An initial key factor is to ensure that they are the right size company and team for your current company or your ambitions for your company for the immediate future. You want to choose a company that knows your industry and is passionate about it or at least a company who are fully willing to learn about your business and throw themselves into learning all about it. They should have contacts with the press relating to your field. You want your PR Firm to be in the neigborhood. They will therefore understand the community that you are based in. You will need to get on with the team and trust them, you will have to work closely with them and tell them some of your business secrets!
You need to establish how your PR company are going to prove to you that they have made a significant impact upon your business. Ask them to fully explain the metrics that they are going to use to do this and in what time frame they can expect noteworthy results.
Once you have looked into these factors and perhaps formed a short list of who you might like to employ for your business then you could consider the following factors to enable you to reduce the list further. You could ask for references from other businesses who have employed them, you could look for comments on their website or Facebook pages. You could give the referees a call and explain your situation and ask them for their thoughts. You could ask them who did the best work for them at the company and what tips they could give you for working with them.
The next step is to get as many of your staff involved in the process as possible, have a meeting day with the few PR firms you are considering get your managers viewpoints. Ask everyone in your company to confirm what they think you need to achieve through your PR exercise. What are your main aims and target audiences for example. When your colleagues leave the meetings, you will be able to tell by their body language and general demeaner how it has went. If they leave the meeting chit chatting about the process and they seem all enthused, then it is likely that this is the company that you should go with.
Have follow up meetings with your staff to ask them to share their views and opinions on the firms and how they think they will fit, or not fit, with your business and what you are trying to achieve.
When you approach the firm make sure you can give them enough information about who you are, what you need them to do for you, in terms of achievement, have a timescale for when this will be achieved by and when you want to see the proof that they have made a difference.
In the end go with your instincts, they are usually trustworthy.