You can download an addon for firefox that will highlight nofollow links. It's been talked about here before. I use SEO for firefox. Just look through the addons on their website.
Either you can use a firefox addon, like noDofollow ( check the find high pr posts link in my sig to see how this works) or, you can do it manually. its very easy. Just highlight someones signature link on that forum, right click and click on view selection source, then check the source to see if there is a rel="nofollow" present. If there isn't , you can safely say that the links are dofollow. For example, I highlighted DrKnutritions signature link, and ivewed the source, this is what i got. Reviews of <a href="http://www.fatburningweightlossprograms.com" target="_blank"> Weight Loss Programs</a> Code (markup): In the a href, there is no rel=nofollow, which meas this forums links are dofollow. Hope this helped you.
I use the seo book toolbar for firefox and also flock, I think is the must have tool for such a things
In Google Chrome it's easy. Highlight and right click a link -> Inspect Element -> scroll down if needed. It's highlighted and will show you if it has nofollow on it.
If the signature link not contain nofollow and the thread i near to th emain category page can say will give some link juice
There is a very cool Firefox plug in that will tell you what links are dofollow and nofollow on a page.