This is actually the part that bugs me about the Red Wedding. It's really one of two cases:
1: All the Bolton/Frey troops were made aware of the plans for betrayal well in advance. From the writing, this appears to me to be the case. And if it were the case, how could there not have been one loyalist or financially enterprising individual who would have alerted Rob and company?
2: Bolton and Frey troops were not made aware in advance of the plan. As Lady Qohor points out, then they would have been just as drunk and unarmed and unarmored as the Starks. In addition, in this scenario, I would expect that the event would have been far more chaotic (not that it wasn't) as many of the Frey and Bolton personnel wouldn't be able to properly receive and process the new orders.