google and educated guesses on what some of the words mean/refer to. Thats what taught me. if its "block Id conflict" then it will say something like block id xxxx is already taken... same with item id's. (that was the main reason for crashes before the universal configs for me.
Then in log files that I come across nowadays, its more like "blahblahblah something liquid something...." then a few "lines down buildcraft something." this means that something is going on with an buildcraft block probably related to liquids.
Basically, just look for key phrases. Some of the lines displayed in the log file are almost completely irrelevant to the average user. For example, 99% of the lines where it will say initialized---->added---->available or something like that are completely useless, very rarely do they show issues (I think this is more for mod developers use when they are building the mods and testing them out)