Check they are insured
A reputable roofer carries public liability insurance, which protects you and your property if something goes wrong. Ask to see it, and do not be shy about it, a professional will be happy to show you. Working at height is one of the higher-risk trades, so this matters more than it does for many jobs around the home.
Look for clear, written quotes
Good roofers put their price in writing and itemise it, so you can see what you are paying for. Vague verbal figures, pressure to decide on the spot, or a reluctance to commit anything to paper are all warning signs. A written, itemised quote also protects both sides if any questions come up later.
They explain the problem and the fix
A roofer who knows their trade can explain, in plain English, what is wrong and how they intend to put it right, often with photos of what they have found up on the roof. If someone cannot or will not explain the work, that is a reason to be cautious.
Ask about guarantees
Quality roofing work should be backed by a workmanship guarantee, and the materials used often carry their own manufacturer warranties on top. Ask what is covered and for how long.
Be wary of cold-callers
Be especially careful of anyone who knocks on the door claiming to have spotted a problem with your roof and offering to fix it there and then for cash. Genuine local roofers rarely work that way. Take your time, get more than one opinion, and choose someone you can check up on.