How do I know fire stopping was done properly?

Ask for the record, not the reassurance.

A good job leaves a schedule. Each seal has a number, a location, the product used and a photo.

The photo has to be taken before anything is closed up. A seal behind a new board cannot be checked later.

Look for labels. A tag or sticker on each seal means the next person to put a cable through it will know what is there.

Ask whether the installer holds a third-party certificate for fire stopping. Many assessors and insurers want one. Ours are done by our BM TRADA / FIRAS-certificated contractors.

Walk a few seals yourself. If you see a gap round a pipe, bare duct at the wall or foam with no label, say so before the crew leaves.