Most people go about doing it in a logical fashion. Do they trust the person they are talking too? How long has that company been in business? How long has that salesperson been at that present company? If they have been at 4 companies in 10 years you might want to think twice on giving them a rose. There in it for themselves. How knowledgeable are they about printing?
Try meeting the friends approach and ask for some references but also realize they aren’t going to give you a bad one. It at least gives you a chance to talk to someone that deals with the company like those local banner printing. Samples are always a good idea but any printer can pick good samples just like they are going to be on their best behavior on the first date.