HI Glen,
They give you a sticker for your boat + one for your trailer. The rangers, Jerry & Rohan, usually have a good idea who has & hasn't got a boating permit.
If you don't have the sticker on the trailer, they usually leave a note on your windscreen as a reminder. They drive around in a 4x4 with lights on top & on the water they have an old white cat with twin 70s on it, Jerry sometimes scoots around on a jetski as well.
There is a manned ranger station just below the main wall at Somerset township, you can get a boating permit there. Alternatively you can get them from the Wivenhoe info centre.
Cheers,
Fitzy..