How to predict if you can achieve results for ecommerce clients (fb ads)

I run an Agency doing Facebook ads and we want to get into the e-commerce niche. How do you assess whether or not you will be able to achieve good results for a client's business before you take them on as a client? Like what traits in a business do you look out for? What numbers are indicators for good facebook ad performance potential?

Ps. I know you will never be 100% accurate in predicting that, but you can get close.

Thanks for your help!