You are required to have a sticker showing the wattage
They are available on
ebay
I might be wrong, but I believe the one which says '250W, honest' is the best seller.
Testing is not a simple matter because the legislation calls for a maximum of 250W 'nominal', a term which is very difficult to tie down.
All motors peak at twice or three times their nominal rating, so at what output does a 250W nominal motor become illegal?
The speed limit is much simpler to test and enforce.
If the bike will do over 15.5mph without pedalling, it's illegal.
Cops can get those simply by following with their video recording - no need for messing around with testing.