I'll assume that the question is about partial menus, not a custom main or enterprise menu.
At the command line you can type in
(findfile "NameOfFile.cui")
If your not showing the command line then you will need to hit F2 to get the answer.
That will usually give the correct answer, but not always. It is possible to have a partial menu loaded either with a hardwired path or without one. If the partial menu is outside AutoCAD's search path when it is loaded, its hardwired path will be stored in the main cui file. If you later copy that menu to the search path while AutoCAD is running, the version in the hardwired path will be used in the current AutoCAD session, but on the next AutoCAD startup, the hardwired path will be stripped and the version in the search path wil be used instead.
You can find the paths AutoCAD is actually using right now easily enough within the CUI command:
[-] Main
:
:
[-]Partial CUI files
[+]Custom <----- Click here and look at Properties on the right
However, if you move files around or change AutoCAD's search path, those paths may change. The easiest reliable way to find the paths AutoCAD will use to find the menus is to open the main cui with Notepad or similar and look at the <PartialMenuFile> tags. If the search path is being used, there will be no hardwired paths in there and the (findfile) suggestion will give the correct answer.
However, if the question was about the main menu rather than partial menus, then the answer is this:
(getenv "MenuFile")
Similarly, the enterprise menu can be found with:
(getenv "EnterpriseMenuFile")