The capability to do RAIM checking is (or isn't) in the receiver, yes. Few handhelds have it in any form, and none I know of have the proper version which requires an encoding altimeter input, and manual setting of QNH.
Whether you can get RAIM at a given place and time depends on the satellite positions. This can be checked computationally; here is one URL I had saved from a while ago
http://augur.ecacnav.com/
However I don't think RAIM has much relevance to PPL-type en-route flying, and one can hardly use GPS for approaches, as there aren't any in the UK.
Incidentally I haven't seen a RAIM warning in 300 hours (2 years' flying), with a KLN94B. If somebody sees something a lot worse than this, they might consider getting a better avionics engineer
