there's a freebie out called inspector. it's pretty good imho, as you can set it in many different ways, e.g. it shows you how many consecutive samples are clipped.
it's a difficult subject as far as i am concerned.....theoretically, as long as you can't HEAR any clipping you should be fine. however, that doesn't mean it's not there.....it depends on the quality of your monitoring chain, and after that on you ability to actually hear fine nuances.....so, it might be that even though YOU don't hear any clipping, it might be obvious to someone with higher quality audio gear, and/or better ears.
on the other hand, many people out there can't tell if somethings clipping even it's really damn obvious......and as someone already said,even supposed "pro" mastering jobs will have more or less obvious clipping.
so, while it's a good thing to strive for the very cleanest mastering you can achieve, it's not that much of a big deal, and you can get away with a fair bit of clipping until the music is really starting to suffer from it.
and at the end of the day, that's what it is about: the MUSIC.
if the music sounds fine/great, it's not too bad if it's actually clipping in a technical sense. it'll only be apparent to real audiophiles with high quality gear, and even then you got to ask yourself: does it damage the music?
the point i'm trying to make is that you should just strive for a clean mastering job that fits the music, or at the very least doesn't do any harm to it. as long as the music sounds great, i feel that you shouldn't worry that much about the technical side behind it. and the better your monitoring chain and your ears are, the more you make sure that as many people as possible will feel the same about the music you created.