Well, unless you're actually the designer of the system, it's unlikely that you'll be able to look at a set of timing diagrams and say to yourself, "hang on a minute... the data at this point is not correct". If you're looking at a system someone else has designed, usually all you'll be able to determine is that yes, varying data is present on each data line or no, varying data is not present on a particular data line. Usually, if you have a set of varying data present on a data line it's safe to assume it's not faulty. You don't know that the data is correct, but the fact that data is there (and the pulse train varies) is usually enough to assume all is well. If you were talking about a system which you'd personally designed (which wasn't working properly) you'd be able to use the timing diagrams along with your own design data and IC data sheets to determine whether all the required control lines were being operated properly and with the correct timing. Timing problems can be amoung the most difficult problems to pin down for an engineer or hobbyist.
If a data line appears inactive for a prolonged period of time, it's possible (although not definite) that it's faulty. If it's stuck low for the entire capture period, it could be that this particular data line is short to ground - switch off the equipment and check with a meter. If a data line is stuck high for the entire capture period, it could be that the data line is short to VCC - switch off the equipment and check whether this is the case. On the other hand of course, it could be quite normal for that particular data line to remain in any one state for that particular capture period!
Digital fault finding is probably the hardest kind there is. Sometimes you find a fault (an i/o pin short to ground for example) but even then you're unable to pinpoint the exact IC that's causing the problem. For example, if you have IC1 which outputs information to IC2 via a set of i/o pins, and one of those pins is short to ground, how do you know whether you've got an internal problem within IC1 or IC2? The only certain way to tell is to remove one of the ICs (or at least remove the solder from the offending pin on one of the ICs) and see if the short dissapears. That technique is fine, but what if you're talking about a faulty data line which interacts with 3 or 4 different ICs? Which one is faulty then? And what if all the ICs are SMDs or worse... BGA packages? It really is very difficult
Brian