Get 2–3 written quotes. A good quote lists the materials (shingle brand and line), underlayment, tear-off vs. overlay, cleanup and timeline. Vague quotes cause disputes.
Ask about the warranty. Manufacturers warrant the shingles; the roofer should warrant their own workmanship in writing, for a set number of years.
Check insurance and WSIB. Roofing is high-risk work. Ask for proof of liability insurance and WSIB coverage before anyone gets on your roof.
Be careful with big deposits. A reasonable deposit is normal, but paying most of the job up front leaves you with no leverage. Tie payments to progress.