When I first started working on computer we had a dram tester. Simple little box with a button for testing and a knob for speed. Lock in the chip to set the speed and hit the buttons. Simple red / green light tells if it passed or not. I found out by pushing the speed up a tad more. It weeded out bad or questionable chips. Luckly this was at the time dram was cheap due to being replaced with simms. I'm looking around just in case I need one to trouble shoot the ram in my Apple II+ When I get it. I found this on e-bay.
Commodore 64 (and more) memory chip tester - 4164/41256 DRAM