Ask what access is included
A quotation should say whether ladders, a tower or scaffold are allowed for. Two prices are hard to compare if one includes access and the other does not.
Make sure the proposed work is the same
One roofer may price a local repair while another has allowed to strip a larger area because they suspect wider fixing or underlay failure.
A vague one-line quote leaves too much open
The area of roof, method and main materials should be described well enough for you to understand what is being paid for.