How do I tell a fair extension of time claim from an inflated one?

Look at the dates first. Did the contractor give notice within the contract period? Did the event happen when they say?

Check the programme. If the task had float, the delay may not move the finish. If the contractor was already late, part of the delay is theirs.

Watch for overlapping events. Concurrent delay can earn time but not delay costs, so check the contract.

Ask for the programme before and after the event. An independent review of a large claim often pays for itself.