From my experience, sitting in a session is no problem for a newcomer. Most sessions welcome new instrumentalists. It is probably best to sit somewhere out of the front line so you can see what is going on and not be too conspicuous yourself if you are unsure of the tunes they will play.
I have in the past left instruments in the car to check out what sort of music is played - I don't care to play fast Irish reels, so that would ensure I left my concertina where it was. If it looks like the sort of music you like and can play, it wouldn't hurt to ask who organises the session and ask to join in. You might get a chance to be introduced to other folks then, and find out about other sessions that aren't advertised. Phoning up in advance is also a good idea if you are sure that the music is exactly what you are looking for.
If you don't know a tune, it doesn't hurt to play quietly to get the feel of the piece, provided you can only just hear yourself. Not playing at all during the evening could be mistaken for being too critical of the musicianship!
A word of caution - if they ask you to lead a set, make sure it is something you know absolutely note-perfect. No-one likes to have several false starts at a tune.
Last of all, enjoy yourself - that is what it is all about.
Mary