Its supposed to go something like this
"Call sign, Vessel Name calling for a radio check on channel XX"
Then if someone hears it they are supposed to let you know how you sound, and what general area they are in so you have an idea how far your transmitting.
But most of the time around here its just someone randomly saying "can I get a radio check" and someone calls them back if its working.
Radio checks are not meant to be on Channel 16, but can be done on other working channels. In Port Aransas Channel 12 is a good for radio checks, as alot of other vessels monitor it.