You could load a known 125kg onto the ball, five bags of cement on a board, something like that, and measure how far down the suspension goes. Providing it doesn't go any lower, you know it has less than 125kg on it.
Did it with a pickup once, measured the spring compression to know when she was fully loaded. Worked a treat.
Missed to make the point what James said too.
If you load it up to 125 KG, and measure it, if your trailer is correctly balanced, you'll not be anywhere near you ball weight limit.