
Hello there, welcome! 
Is just jumping in social acceptable? Should I send folks a tell?
Jumping in is perfectly fine! Sending a /tell is good, too, and may be considered more polite to some people! Either way, though, you can't go wrong!
How can you notice a roleplayer from just gamers? I know on Ismania I got noticed just by walking around. Made a friend, but he hasn't been on at the same time.
The tidbit about the search info is helpful. I'd also try to learn what FC tags belong to RP FC's. It's also helpful to attend RP events on the server (keep an eye on the calendar and the events forum here). Seeing people walking instead of running is also usually a good sign, or you can just approach people you see already making RP posts. It's a bit of extra work and not everyone has one, but if you're eyeing someone and wondering if they RP, you can search for their character's wiki or a profile on here.

Is just jumping in social acceptable? Should I send folks a tell?
Jumping in is perfectly fine! Sending a /tell is good, too, and may be considered more polite to some people! Either way, though, you can't go wrong!
How can you notice a roleplayer from just gamers? I know on Ismania I got noticed just by walking around. Made a friend, but he hasn't been on at the same time.
The tidbit about the search info is helpful. I'd also try to learn what FC tags belong to RP FC's. It's also helpful to attend RP events on the server (keep an eye on the calendar and the events forum here). Seeing people walking instead of running is also usually a good sign, or you can just approach people you see already making RP posts. It's a bit of extra work and not everyone has one, but if you're eyeing someone and wondering if they RP, you can search for their character's wiki or a profile on here.