In reply to snoop6060:
I think weaknesses show up when you ask the question: why am I not getting up this climb? The simple, go to answer for many climbers is 'I'm not strong enough'. This is a pretty useless answer because it means you'll never do that climb until you've gone away, trained for a good while and come stronger.
Weaknesses don't even have to be physical, very often they'll be mental. For me I know one of my weaknesses is making difficult clips when getting pumped. I could think I need to get fitter so I'm less pumped. But that would be running away from the real problem, which is the fear of falling after pulling a bunch of rope out.
Weaknesses can also be in technique. This doesn't necessarily mean you have bad technique overall, just lacking in some specifics. I remember last winter bouldering with someone who pointed out that I didn't use my heels that much. This was a great insight and it's something I've got better at but still need to work on more.
It's common for many climbers to have poor roof climbing technique. It's not surprising since we tend to do much less roof climbing than other styles of climbing. But the simple reason people come up with is 'I'm not strong enough'. That may be part of the problem but it gives people the excuse to stop trying and never actually get to grips with the technique side.
Bouldering with other people of around the same standard is good way to spot weaknesses and differences in your styles of climbing. You might notice that they're getting up all the steep problems whilst you're better at slabs (or vice versa). Working on styles of climbing you're worst at is working your weaknesses. Try to seek out routes or problems you find hard but that have relatively low grades, ie. a grade you normally find easy. Try to find what forces you out of your comfort zone and that will be a weakness.