1) How long have they been in business?
The barrier to entry in the janitorial business is low and there are a lot of companies out there. This is an easy thing to check and provides some measure of stability.
2) Do they have sufficient insurance?
It is amazing at how many customers never ask to see a copy of our insurance certificates. Make sure your service agreement explicitly states your insurance requirements and ask that your company be added as a named insured.
3) Are they registered with the BBB?
The BBB is a neutral 3rd party that handles complaints about businesses and ensures they operate professionally. Companies who pay to have themselves listed with the BBB are confident that they can make customers happy.
4) Do they provide additional services?
It's nice if your janitorial company has a full line of services. It is very convenient and often less expensive to have the same company clean your carpets and wax your floors.