Experience with your product category. A studio that shoots apparel all day long may not have the lighting setup or surface inventory to handle glass, liquid, or reflective packaging. Ask to see work that's similar to what you're bringing in, not just their best shots across every category.
A studio space they own or control. Renting a shared space day-to-day creates variables: different lighting rigs, different backdrops, different setups every time. A dedicated studio means consistent infrastructure, consistent results, and no scramble on shoot day.
Clear pre-production process. The best shoots are planned, not improvised. A studio that asks detailed questions before your shoot day will deliver better results than one that figures it out on the fly.
Retouching included, not quoted separately. Final images should be delivered clean and ready to use. Make sure retouching is part of the quote, not a line item surprise after the fact.
Straightforward pricing. If a studio won't give you a clear number until after the shoot, that's a red flag. You should be able to get an itemized quote based on your product list before a single light is turned on.