You just need to be running the same mod as the server to join. In the case of LFS Tweak with it's infinite variations, you'll need the exact preset files in use. Not the easiest thing to co-ordinate.
I've just scanned through all ~800 licenced servers online though and I can't see any marked as modified?
Agreed, all servers with mods should state it, that would be the case of the mod writer? Mind you, would that include things like the CTRA X system and Grid-calling mods?
But definitely, if you're looking for a tweak server, you'll have OOS errors constantly unless you have the EXACT setup the server host has. Generally tweak is a SP thing, I guess. Can't see the desire to use tweak myself, just run a different car/class...