How do I properly measure the current of power supply?

Hi,

I just converted cheap 450W ATX power supply to 12V one for RAMPS 1.4 controller.
The current output of this unit is total +12V@30A (+12V1@14A, +12V2@16A).

I merged all +12V wires into single wire, almost same thickness of car battery cable, and attached two AWG 16 wires for
two +12V output. I installed 15A blade fuse on the 12V side.




The voltage output of the 12V wires were around 12.11V.


It hasn't blown the fuse yet.

I want to make sure the power supply is okay before I hook it up to the RAMPS board.
Is there a good way of measuring current of the power supply without risk of blowing the fuse of multimeter?
My multimeter can measure up to 20A.

Thanks in advance,
Hughe

PS: A portable circuit breaker unit I made.