Test current capability of a 24V 15A power supply
To test if your 24-V power supply can deliver 15 A of current, you need to get or make a massive load resistor with value of 1.6 Ohm. Better make it 1.5 Ohms, since every power supply must have some margin. 

You can make it out of piece high-resistance wire, or maybe out of a spool of household iron wire. Or you can get 10 pieces of massive 50-W 15-Ohms resistors in parallel (you need your load to hold about 340 W). Many variants exist. Connect the load with thick copper wires (12-14 AWG), and see if your PSU still delivers 24 V.