Yeah, getting an eye for this is simply a matter of studying anatomy, and practicing. In the mean time, let me tell you what I see, and maybe that'll help. Are you using pure greenstuff? If so it doesn't sand/grind/file away too easily, but at this stage that's fine. Here's what I suggest:
1. you've got the muscle groups marked out just fine. He's a little 'lifesize' right now - that is his limbs are about as thick as they would be on a 28mm model. That's going to make putting a 'skin' over it tricky without him looking like his limbs are a little thick. Another thing that comes with time is an undersanding of putty quantity - you can see it in the legs - the leading one is a different width from the back one, you can see this most clearly just above the knee. Apart from the knee cap which is rather pronounced, that front leg is pretty much spot on. Maybe you could take a little off the rear one in line with it?
2. A view from the side would be handy here - how is his chest/stomach looking from the side? If that bulge is intended to be a slightly larger than ordinary belly, then its an easy mistake to make, but it's a little low - it needs to sit higher to give him room to keep his bits and pieces
3. The left forearm seems a little long in comparison to the bicep section - maybe move the elbow down the arm. Likewise, the left forearm looks a little 'bent' - maybe getting a file on the outside edge of that forearm would sort that out.
4. Lastly, and i'm sorry to do this to you matey, but I think the other thing that makes this a little off (at the moment) is the comparative width of the shoulders - one is 'wider' than the other...no easy answer to this I'm afraid, move the head sideways on the shoulders, or shorten/lengthen one of them by cutting it off and re-pinning it.
Having said all that, don't lose heart dude, it's looking pretty good, better than some people with a lot more experience do...don't sweat it - Make any changes you want to, then settle on your laurels, chill out and have a go at the trousers next week

It'll all feed into the next project!
James