Roms: latest versions of roms and 'outdated' roms ...

I have heard that roms get updated from time to time and that roms that used to run on and older emulator no longer work on later ones.

Now how do you know if a rom (that is no longer working) is incomplete (missing files possibly because of bad source), outdated, etc? How do you know whether you need an update rom or need to get it from somewhere else?

I am reasonably new at this stuff so I still get confused from time to time.

Thanks for your help!

usely mame wil inform you if a rom is not correct, or you can use clrmamepro to keep your roms up to date