If they are wearing their meters in "normal" mode i.e. you can see their details/stats they should be in RP (that doesn't necessarily meant why are in a RP scene...just that they are in character)
(Sometimes people forget to put their meters into OOC mode so if you're not sure you could ask them politely in IM)
If their meters is in OOC mode or they are not wearing a meter then it is unlikely that they are RPing (unless they forgot to activate their meter of course in which case, follow advice above).