It's a bad sign if you look out your window and the roofer or his representative is already walking around your house. He should knock, introduce himself, ask to-the-point questions about what you want done and listen to your answers. He should also ask questions such as how long the roof has been on the house, and what type of materials you would prefer. He should have samples, and offer suggestions.
Beware the rule of thumb. Chances are, if he's been in the trade long, the roofer already has a general idea how much the job will cost. But he should also know that surprises will upset you and potentially cost him money, so he should conduct a thorough examination. He should have his own ladders and use them to get up close and personal with your roof. If he wants to take a peek in your attic to examine the underlayment, he's being properly thorough.
The tape measure should come out next, if it hasn't already, and again he should be thorough. The roofer should also be totaling up the extras, such as vents, valleys and flashing.
During his inspection, the roofer should be respectful of you and your property. Afterward, the ladders and any other equipment he used should be stowed before he works up his estimate. Even if he doesn't do the work himself, he represents the people who do, so be sure you like what you see.
If the roofer presents his estimate and disappears, he might do the same thing after he does the job, leaving you in the lurch with problems. He should take the time to explain the costs involved, answer any questions you have and again offer suggestions if there are variable options.
When your questions have been answered, the roofer should take his cue and say goodbye. He shouldn't linger and ask about other estimates you've gotten or pressure you into a decision. Roofing is a tough, competitive business, but you have a right to expect courteous, professional behavior.