Get an actual list of customers with addresses and phone numbers. Contact several of these references and get honest opinions. Visit web sites the designer has created. Everything should work properly and look like it belongs. Check the spelling, punctuation and grammar.
Errors are evidence of slipshod work.
Contact the site owners and ask for their opinions of the designer. Examine print materials carefully. Every element should be part of the whole. Again, errors in spelling, punctuation and grammar are evidence of slipshod work. A poor printing job is evidence of either poor design or substandard printing.