Do you have work in my industry?
Anyone can show a highlight reel. Ask specifically whether they have done work in your category and whether you can see it. A manufacturing capability story and a restaurant commercial are different skills. You want proof they have solved a problem like yours.
What is your pre-production process?
This is the tell. If the answer is vague, the video will be too. Strong companies describe a real process: discovery, narrative development, scouting, interview coaching, a shoot plan. Pre-production is where a video is won, so ask them to walk you through it.
Can I talk to repeat clients?
References from clients who hired them more than once are the cleanest signal of reliability. One-off projects can go fine by luck. Repeat work means the process holds up. Ask for 2 or 3.
Who owns the footage, and what is included?
Confirm you own the final video and the raw footage if you want it. Some companies retain raw files to lock you in. And get the scope in writing: how many revisions, how many deliverables, and what triggers an extra charge. Surprises here are the most common source of friction.