You could try a jumper between any two wires on the reverse connector and slowly apply throttle to see what happens. If you want to be extra safe, use a 1k resistor instead of a jumper, but those connections are made to be grounded. One wire should be ground. You could also try measuring the voltage on the 3 wires with the controller turned on. Measure vs. battery negative.
Some controllers will be speed limited in reverse more than forward.
"One test is worth a thousand opinions"