Unsleeved bars I inspect once every few thousand miles. Basically whenever the bar tape or the cables need fiddling with seems like a good time. I take the view that clamps with sharp edges are to be avoided at all costs. I also worry about the texturing that is applied to the clamping region of many bars; I think in many cases it is exactly the kind of thing that helps fatigue cracks to start.
Sleeved bars cannot be inspected in the same way. I will tend to scrap a set of bars where there is obvious deformation of the clamped region; it is sure that the bars themselves are deformed (and potentially damaged) beneath a deformed sleeve. I don't especially like Cinelli 1R stems, because I've seen too many mangled handlebars come out of them. I don't minds a few scuffs on the sleeve (that is what it is there for) but I am cognisant of the possibility that they may hide cracks. If the sleeve is uncracked, and the bars are otherwise not deformed, don't flex unduly, don't creak, and are not corroded then I will assume that they are probably still OK.
If the sleeve is a good fit on the bars, then, in the absence of abnormal stress-raisers beneath the sleeve, I would say it is very unlikely that the handlebar would fatigue beneath a typical (marked up, with built in stress raisers) sleeve, without the sleeve istelf cracking first; if the two share the load, then the sleeve should see higher strains than the bars beneath.
However if they are just very old, I won't carry on using them on a bike that regularly sees rough surfaces, hard sprints and/or hard braking.
I'm not right keen on bars with abrupt steps in them, or bars in high strength materials which have grooves etc in them (and holes....? No way.).
I don't think I'm very hard on handlebars; some folk appear to be permanently wrestling with them as they go down the road, but my hands rest lightly on them except when honking etc, and I avoid that where possible these days. Possibly that is why I've been lucky myself; I've scrapped a few handlebars that I didn't like the look of, were pranged, corroded or that were definitely cracked, but I have (touch wood) yet in use to break a set of my favoured handlebars which are older Cinelli, GB and ITM models, all sleeved, no grooves.
I have crashed heavily through losing my grip on the bars, and I'd imagine that the same thing could happen if they broke.
In case anyone is wondering if handlebars can corrode, if you work hard on your bike, sweat gets under the bar tape and all kinds of horrors ensue. Check this link;
http://www.pardo.net/bike/pic/fail-001/FAIL-160.htmlmy hands sweat too and I have scrapped bars that looked (a lot less) corroded than this. Since I realised this was an issue I have taken to adding a waterproof base layer to the handlebars when taping them up.
BTW you can test any normal handlebar for the presence of a through-wall crack by seeing if it leaks air or not. Since a crack will go through-wall in short order, but then might take a considerable length of time to cause failure, regularly checking for cracks in this way provides a reasonable assurance that the bars are not about to break.
cheers