Or you can disconnect the 7 pin while hitched and the tow vehicle is running. Then pull the breakaway pin. Get in the TV and put it in drive, remove foot from break, the trailer should keep the TV and trailer from rolling. If they don't roll the trailer brakes have activated and trailer battery is supplying power. Along with pulling a tire and looking at the brakes on that wheel this is what they do at the vehicle inspection station to show the trailer brakes are working, the breakaway is working and the battery is working when I get my annual state trailer inspection.
If the trailer does move, the brake controller knobs or levers may need to be adjusted or there is a problem with the breakaway, the brakes or the breaking system.