All great questions that a lot of people don't think of asking. Newton and LS have done a very good job of addressing many of your questions up to this point, but I'm here in the capacity of a CA Professional Land Surveyor to do some minor tunig to the previous answers. I'll start with asking for some clarifications.
The 1987 date keeps popping up, but I'm confused as to its particular significance. Is 1987 when your mother purchased the property, when it went into probate, or both? Was the probate, and the time the fence was moved recent or in 1987/88?
Do you have knowledge of where the fence was prior to being attached to the garage? Do you know on what basis the neighbor moved it? Does he have some belief that the true property boundary is where the fence is now, or is it a matter of convenience or some other motive? Have you and he spoken about it or are you speculating based upon your impression of him?
I'll come back later to tune up my answers a bit pending the answers to those questions.
So far, you have had three surveyors (including me) and no attorneys answer here. Maybe an attorney will come along and chime in soon, but so far there isn't really enough info or answers to speak much to legal issues. But as a surveyor who sees these issues commonly, I agree fully with Newton and LS that you have little basis for challenging the fence without first having a survey performed.
Answers to your specific questions:
1. A real estate lawyer would be a good place to go for a recommendation if you're pretty certain that you will need legal representation. If you have knowledge the fence was moved, if zero setbacks (buildings on the property line) are not common in the area of this property, it's a good bet you will need legal representation at some point.
2. If you are finding a surveyor yourself, bear in mind that in CA, the practice of land surveying is much broader than it is in many other states. Some states recognize the licensed activities as being only boundary surveying and some mapping, meaning that only licensed surveyors can do those, and other kinds of surveying can be performed by pretty miuc anyone whether they have a license or not. In CA, those are included, but so is geodesy, photogrammetry (making maps from aerial photographs), construction staking, and a few other types of surveying. Accordingly, there are many licensed surveyors whose practice is primarily one of the other types of surveying, with boundary surveying either not being a part of their practice or being a minor part of it. In many cases, surveyors whose practice does not include boundary as a major portion of their business don't have a very good understanding of the records, physical evidence, laws and other issues which need to be considered. Find someone who has boundary as a major portion of their practice. The host of this site has articles on boundary doctrines based upon occupation. After finishing this post, I'll find them and link them for you. Make sure the surveyor you hire can explain not only what forms of evidence he or she will consider beyond your deed, but can also satisfactorily articulate the concepts of Adverse Possession and Acquiesence, how they are similar, how they differ, and how each may apply to your circumstances. You won't be looking for legal advice, but an assurance that the surveyor you are speaking with has an understanding of boundary concepts beyond the mathematics gleaned grom the dimensions of the deed. A surveyor may be quite competent and succesful in other areas of surveying, but that does not necessarily equate to high competence in all areas. Boundary is an area that many perform, but relatively few adequately understand because so much of it is legal in nature as opposed to mathematical. A competent boundary surveyor is not going to be the cheapest of those you speak with, and may be the most expensive. I'm certainly not implying that a high fee is a sure indicator of a competent surveyor, only that competent surveyors tend to know the value of their service (and deliver) and less competent ones tend to not recognize all of what should be considered, viewing it more like a staking problem than a quasi-legal one. It will be up to you to determine if the high price is based upon a more diligent effort of identifying and analyzing evidence, or simply a desire to make a bigger profit. Thus ends my standard mini-lecture on the difference between fully competent boundary surveyors and others. I hope you found it useful.
3. Here is a basic short list, but as LS stated, there isn't enough time or room to get comprehensive here. I have about 10 feet of bookshelf filled with books that discuss the answer to this question. Short list: your deed; recorded survey maps of properties in the vicinity, which may include subdivision maps and other boundary survey maps; the physical locations of survey monuments in the area, including those which may exist at the corners of your property and others which may be some distance from your property; the locations of existing physical improvements on or very near your property; local government survey and street records, including surveys, right of way plans or improvement plans by the Dept of Public Works); private survey records reflective of unrecorded work in the area for which your surveyor found points; your neighbors' deeds; and the list can go on based upon local circumstances.
4. It might. But the surveyor will not assess safety issues. He or she may be aware of local setback requirements that exist or were common in the era when your property was developed. In many neighborhoods in urban or suburban areas, it is not particularly common, but is not exactly rare either that zero setbacks, particularly for garages and other outbuildings are or were allowed for some neighborhoods/developments, or that a variance may have been granted to allow it. Don't discount that until you've checked it out. If such setbacks or variance is present in your case, look into any related zoning ordinance which may address maintenance of structures built at a zero setback. Even if your garage was so built, you may have some relief with respect to your neighbor being able to use the wall to hang tools, and for you to perform regaular mainrenance. While zoning codes are usually devised with the fire code in mind, they are not the same. Fire departments care about access, but may not care particularly if that access is fully on the property burning or on a neighboring property. Where zero setbacks do exist, you will commonly find that they may not exist on both sides of the property in a single family residential neighborhood where the homes are not joined by party wall (like a townhouse).
Your surveyor may be able to help you research these issues, or may be able to give you some local guidance to do your own research. Your attorney may also be able to give you a good recommendation of someone locally who specializes in that type of research.
I'll go look for those links to the articles I mentioned now.
Here's the article on Acquiesence: http://www.expertlaw.com/library/rea...uiescence.html
And here's the one for Adverse Possession: http://www.expertlaw.com/library/rea...ossession.html
I'll check back later for your answers to my questions.