Ask them if they see any potential issues with manufacturing your CAD design, including whether they think any weld marks will show through the metal, whether any bend radii are too tight, whether they can foresee any issues in assembling the parts at the end (you should send assembly instructions, or work with them on them) etc.

Ask them what their prices would be for 1, 10, 500, 10k, etc. units, so you know how well you can scale up with them if needed.

If you're in initial discussions with them, find an existing product that exemplifies the quality you would like to get from the shop, and take photos of the example. For instance, it you want the quality and finish of an aluminum Macbook, use that as the example. Make sure to include the color and finish instructions in whatever files you send them.

Ask them for a single sample before a full order, and how much that sample would cost. You're going to want to have a "golden sample" that you approve of, that they will try to match in all subsequent copies. It's also good, if possible, to have "limit samples" which provide high and low bounds of the product's final appearance.

Let me know if you need any more help.

Best of luck.