replace old flat roof

diyer2010

Hi there flat roof experts,
The house is 1910's , the landing is flat roof and the extension for the ground floor bathroom and kitchen flat rooves might be built after 1950's.
How do you find it out that your board under the old flat roof need replacement? and how do you find it out that your old flat roof has any insulation?
There isn t any leaks in any of the flat rooves or any stain of water on ceilings even though the flat rooves area old but the flashings where the join to the main house are not good condition.
Any advise will be very much appreciated and b. regards,
Diyer2010