What should be on my asset register?

Enough that you could answer an insurer’s questions without relying on memory:

• What it is - make, model, and serial number where there is one.
• When you bought it and what you paid - with the invoice stored alongside.
• Where it lives and who has it - especially for anything that leaves the building.
• A photo - the fastest evidence there is, and it takes seconds.

One afternoon builds it. Keep it somewhere that survives the event you are insuring against.