I've been having fun learning PSK31 and JT65 - it's keeping me busy, and I've learned what they look like on the waterfall over time.  My question is this - there are so many other modes out there...  how do you identify (sample?) and then decode an unknown signal / mode that you see?  On 30 meters, I see a lot of different stuff coming across on 10.139.  

Or, is it as I assume, and you just have to work those other modes at their own normal frequencies and learn to see them by experience?

There are numerous sites covering modes of digital.  Experience of both sight and sound helps.
Digital modes sight and sounds.

I think a lot of it is  just experience. There are a few website out there that show what the different modes look like and sound like.

If you learn the PSK modes, JT65, RTTY, and Olivia and its variants, then you will be able to recognize 90% of what's out there. There are quite a few more modes than those I listed, but you will pick them up if you use them.

I'm on my iPad at the moment, but if nobody else posts the links with digital descriptions then I'll do so tomorrow.

Very good - I'll start there!  Thanks!

This one is pretty good digital mode samples