The main thing to check about any firm that says they can help with your seo is where do they rank for their main keywords, did you find them on the first page of a search engine or did you find them through a paid advert such as Google adwords.
If you found them through a search engines organic results then this is a good indication that they know what they are doing and can possibly help you get more traffic to your website.
The next thing I would suggest is will they look over your website and
clean up the onsite seo, and will they do keyword research to make sure you target keywords that will bring you traffic if they don't then this is a big sign that they are no good for you.
The main thing to stay away from is spammy links always ask them for examples of their work and how they achieved the rankings.
James
