The questions that separate the quotes

"Can I see a job you finished five years ago?"

Recent work shows tidiness. Older work shows competence. Ask for the older one.

"What is your licence number, and who holds it?"

Then look it up. You are checking that the licence is current and that the name on it matches the name on the contract you are being asked to sign.

"What underlayment, and how are the valleys done?"

This is the question that reveals whether you are comparing the same roof. Underlayment, ice-and-water placement, drip edge and valley treatment are where quotes quietly diverge.

"What does your workmanship warranty cover, and for how long?"

Material warranties come from the manufacturer and are conditional on correct installation. The workmanship warranty is the contractor's own promise, and it is worth exactly as much as the company behind it.

"What happens if the decking is rotten?"

You want a rate, not a reassurance. Some rot is likely on any older roof and the rate is the only thing that keeps that predictable.

"Who is actually doing the work?"

Ask how long this specific crew has worked together. Roofing companies keep their name and change their hands more often than most trades.

In short

Six questions, three contractors, one afternoon. It is the cheapest insurance available on a job this size.