Find out how much current a mains adapter can supply?
You could gradually load up the output with say 100 ohm 2 watt resistors or anything else that you have on hand .Measure the volts and the Amps .At overload point the supply will  start to drop voltage and /or trip out .It may hiccup trying to restart .Set up the load at say 80% of this point and leave the unit running for thermal evaluation .Back the load off further if things are too hot .