You’d find the maximum noseweight for your vehicle’s tow hitch in the vehicle handbook (owner’s manual), in the towing/technical specifications section.

For UK driving theory test style questions, “in the vehicle handbook” is the officially accepted answer.

In practice, you can also confirm related limits on the towbar’s data plate and in the trailer/caravan handbook, but the primary, test-correct source is always your vehicle handbook.

TL;DR: Check your vehicle handbook for the maximum permitted noseweight, and stick to that figure for safe towing.

Information gathered from public forums or data available on the internet and portrayed here.