how can you tell how fast a proxy server will be other than by adding it and testing it out?

I would recommend either:
1. Testing it out
2. Asking people who've used it

You can't.

You can traceroute to it and compare the trace to previously known data. By analyzing performance along various routes to various targets you can create a database of statistics which will eventually allow you to estimate the speed/link capacity of any given proxy without actually connecting to it. Of course, other factors may slow a particular proxy down, but you can never account for all factors without actually testing it.