Ask for references from their neighbors. Or, if you're really desperate, show up at their current building (if they are in an apartment) or neighborhood and start knocking on doors. It's almost impossible that no one will have anything negative to say about them. Downstairs neighbors are the best ones to talk to. Ask to inspect their current living space, to see how they keep it. Show up unexpectedly. Then you can ding them for all the food wrappers around, the filthy dishes they inevitably have piled in the sink or on the table, the scuzzy bathroom, the nasty overstuffed fridge, the piles of dirty tarps on the bedroom floor, the general stink in the air...there's got to be tons of great reasons to refuse them lurking in their current home.