Ask other meeting planners to recommend companies to bid on your project. Most professional DMC's will be members of these associations. Ask your hotelier for DMC recommendations.
Many hotels now have their own internal destination management services.
If not, previous clients often give hoteliers their opinions of the companies they have used. Give a deadline for proposals, and adhere to the deadline. If a proposal is late, that should be a red flag. In comparing proposals, look for value. Remember, the cheapest may not be the best deal.