As I've written before, participating in online discussion forums is a worthwhile activity for bloggers and webmasters. You can learn lots of new stuff as well as contribute what you have already learned and thereby help others. It's also a great way to develop material that you can rewrite substantially at another time and post to your blog or an article directory. And if the forum allows signatures that are dofollow, then you will be building links to your site with your chosen keywords in them, thereby helping to optimize it for search engines.
If you do want to get SEO benefit for your participation, then obviously it's worth checking a few threads on each forum you've Googled to see if the posts do have dofollow sigs in them. Also check how many posts the signature owners have racked up. Sometimes the forum administrators only allow sigs after a certain number of posts. It might be 10, 25, or sometimes more. (The massive, active forums at Digital Point, for instance, require you to make 10 posts if I remember correctly.)
Also check the page rank. It's not hugely important. But if the forum has been around for a while yet still has no page rank at all that's not such a great sign. It probably won't hurt you to join it, but it won't help much either. A really high PR forum will tend to help your SEO more (although this is not guaranteed, since there may be just so many people posting in it that this factor becomes almost meaningless). Still, it's something to consider.
The other main factors to look for relate to how large and "alive" the forum is. The problem is that a lot of forums are too small, inactive, dying or dead already! Needless to say, you want to avoid these.
So check the forum home page and look for the numbers of posts and threads. The more the merrier, of course. But also take note of the proportion of threads to replies. If the ratio is close to 1:1, then it's an inactive forum. But if it's something like 1:7 then it's buzzing. Also check the dates of the most recent posts. If there are lots of threads with "today", "yesterday" and recent dates listed, with posts by many different members, then that's another very good sign.
You should also check for pharmaceutical or adult related spam. Some forums are just saturated with the stuff. If there is just a little bit there - even in an otherwise fairly active forum - it probably means that it's already dying, or that the administrators are bit lazy. Either way, its best days are probably behind it and it should be avoided.
Of course you would inevitably learn these things a little while after joining each forum even if you didn't look for them. But by keeping an eye out for them beforehand you can save quite a bit of time in the long run.