First, ask your colleagues and friends who they have used. Next, research each company's website for services you need or if they have clients similar to you. Next, make direct contact and explain your needs to see if they think you are a good fit. Finally, ask for references, and call every reference they provide.
You should do some thorough research via the internet. You can check with friends and family for references. Check with co-workers or others that have or are doing what you need to be done. And don't be afraid to network and ask around. Check with the Better Business Bureau to see if they have any complaints.
1. Check employment history of the firm
2. Support given by the firm to the candidates to find good projects
3. Contact candidates to know how the firm treat them.