My laptop just fell 3 feet and hit a ceramic tile floor. It appears to be working fine and is fully backed up. Anything I need to check / any diagnostics I should do?

I would run exhaustive tests on the Hard Disk. It could have damaged sectors.

Do you think the disk is at risk from impact, even though it’s an SSD? If so, do you recommend a particular utility to use for testing? (I haven’t done any maintenance on this disk.)

Unlike hard drives, SSDs don’t give a rat’s ass about impacts, so you’re safe there.

There really isn’t much you can test though. Things that often happen on impact are hard drive crashes, broken displays, and rattling removable modules (like RAM) and cable connections loose. The first is a non-issue for you, the second didn’t happen, and the third cannot really be tested for.

I would try opening your command prompt and doing chkdsk to check your hard drive integrity

My closed laptop was accidentally knocked and fell about 4 feet onto a concrete floor, it was closed, I thought it would have killed it but it kept going for several years and when it did die it was the wiring between the body and the screen that gave out.

There are no moving parts inside the AIR. If everything still works it should be fie.