Actually, there are inexpensive devices that will test the supply under load. There are a variety of input connectors and an LCD display. Some will support logging to a phone or pad.

I don’t usually need to go to this trouble because the supply in question will almost always have low voltage and/or ripple, even with no load. I like to confirm a defective supply before replacing it because the complaint is usually “intermittent issues” — caused by low voltage. Without confirming the defective supply, I’d need to hang around much longer to verify that the system is no longer intermittent because I’ve identified the root cause.