Hi, I'm looking for the name of that tool people use to test the key weighting on individual keys.
The ghetto tool is called rip-o-meter, and it makes use of the fact that one nickel weights 5 g. However, there are several issues. First, force is measured in Newtons and F=m*g—most people ignore this. Second, it's quite difficult to measure actuation force of most mechanical switches, when it's not equal force required to bottom out, and it doesn't tell you anything about the upstroke.
Precise tools are anything but cheap.
I have several keyboards that are listed as being "all 55g" or "varied weighting 30, 45, and 55 g".
That sounds like Topre or KeyTronic. Search for an old thread about KeyTronic that discusses different ways to measure weighting.
I'm also interested in buying a new mechanical keyboard and I'd like to get the lightest-touch keys possible, but I'd like to verify it for myself instead of relying on the manufacturer's marketing.
Your best bet are either 30g Topre, Cherry MX Red modded with lighter springs (ergo clears or lubed browns may work as well, if you want tactility), or Cherry MY without springs. Goldtouch rubber domes are very light as well.