What can passengers do if they realise their plane is not on the right path?
Okay, so, going with the idea that you have a compass (and also with the idea that this can be a general question, not necessarily about flight MH370).  If you're heading from Tokyo, Japan to Seattle, Washington, USA (direct), then you could be reasonably certain the craft should be somewhere between headings 45 and 135.  So if you notice the craft is on a heading of 270 (ie., clearly you're going in the opposite direction) and this direction has been sustained for say, an hour, I'd think it would be okay to at least ask the cabin crew what's going on.

As with any time you are raising a concern with the cabin crew, you should be polite.  As has been stated, they are used to passengers being a bit nervous and, as a result, they can easily interpret belligerence as a sign of irrational fear.  And, as you might guess, it's easy to ignore someone if you feel they are being irrational.  So, if you want be taken seriously, be polite.  You're far more likely to get an actual answer, or raise a valid concern even.

So, if you just ask, "why are we headed west instead of east" they may have a very simple answer like, "well, the pilot mentioned we might have to take an odd route to avoid some weather" or "this is the flight to Bombay India, how did you get on it?"  Or, they may have no idea and be concerned themselves, so they go and ask their superior.  Thus, goal achieved, you've alerted the flight crew :).  

Either way, so long as you're polite and don't become insistent (or belligerent) that you are correct, I can't see any harm in just asking.  And sometimes asking is what leads to a problem being solved so...  I'd say go for it.