1. Look at the price of the video and the quality. If you realize that it is not balanced, then there is something fishy going on.
2. Check studio's portfolio. All of their videos should be around the same quality and style, it means they have created them themselves.
3. Find reviews about this studio. It is better to use trusted websites like the Manifest or Clutch.
4. Ask everything you want to know: the location of the company, what is included in the price, who is going to work on the project, and how fast you can get the final result.