Go to Guitar Center...buy one...bring it home...try it out...
record with it...bring it back within 30 days, return it for another one...bring it home...etc etc etc.

AB Them....Bring home TWO if you're daring or someone has them to borrow...

Only true way is to listen to them in your environment that you know. Trying to listen to them in other environments will be tough. Your ears aren't tuned for other people's places or store environments.

Opus

Buying and returning is one way. FYI: GC has 14-day/15% restocking fee that they may or may not inforce.

The other way is rent what you want to hear or pay for studio time in a place that has a few of they type or kind your want. Record what ever you plan to use the mic pre with (vocals, ac guitar, drums, whatever) using either mics you own or will be using with the mic pre's and take that home and evaluate it for a few days. Buy the one or ones that tickle your fancy...

goto the listening sessions.com and listen to samples

Hey, I don't think that I was clear.. Sorry.
I plan on doing the buy one try one, return it, buy the next and try it approach. Aside from simply plugging a few mics or line inputs into it, I just wanted to know if there are other standard test signals that would be smart to run through a pre in order to put it through it's paces.
Best regards,
pAndAmOnkEY

Record loud stuff ... snare kick guitar amps .... bass and of course vocals ... listen for headroom issues and saturation / color characteristics ...

Record soft stuff like acoustic guitars other acoustic instruments, listen for noise issues, clarity, saturation / color characteristics ... also percussion ( transient response) .

Try different types of mics with it ... dynamic and condensers ... if you have a ribbon be sure the pre has enough gain to use it. ...

In general just run it through the paces and listen.