I haven't found a real good way to do any "testing" either, but like Becca, try to pick fish that don't look bit sized. I learned the hard way that things that look like food usually will be considered food, especially for New World cichlids. I can remember years ago when I added a group of koi swordtails to a tank, only to have 2 of them immediately eaten and the other 4 being chased relentlessly. And since I had added them to a 180 gallon tank, there was no chance I was going to be able to fish them out before they were consumed. Lesson learned....
Now, I tend to stick with high bodied fish with my New Worlds (i.e. silver dollars) or smaller cichlids as dithers.