Never had disc brakes on a trailer, but a simple quick stop test always worked to verify for me that my drum surge brakes were working---
Either in a big empty-ish parking lot or empty-ish road, get up to about 25 mph, then do about a half a panic stop...enough to slow down real quick without locking up the wheels...
The surge brakes will take a split second to react, but if they're working right, you'll definitely notice it when they kick in.
If you notice a SLIGHT change, could be one side is working and one side isn't. If you don't notice any change, you're probably looking at some repair work...
1988 SkipperLiner Custom 53x14 - Twin Merc 120hp/Alpha's
2002 Regal 2860 Commodore 28x10 - Twin Volvo 4.3/DP's
1995 Sun Tracker 32' Party Cruiser - 115hp Mariner OB
1987 Sea Ray Seville 21' Midcabin - 5.7 Merc/Alpha