As mentioned, the "killer caps" (which is a wholly silly term) are those with the fold down tab. They were the factory arrangement on some Cessnas of 177 and higher numbers, not a 172 or 150/2 that I have ever seen.
There are two short comings of these caps, compared to other simpler ones: There are two O rings which must be maintained. Well meaning maintainers replace the larger outer one, but can miss the inner one. Water will fill the pocket for the folding tab, and run down the tab's shaft into the tank. The other problem can be that a careless fueller can engage only two of the three locking tabs, and the cap does not seal. Though the cap is "on", you can loose 30+ gallons of fuel from the wings of a 177RG in 20 minutes - ask me how I know!
If these caps are well maintained, and properly used, they are perfectly fine. They are the common cap for many larger aircraft, and not subject to ridicule in those installations - probably because of more careful maintenance and use.
As said, if you suspect water in your tanks, drain the sumps (ALL certified aircraft have tank sumps which can be drained) before you move or fuel the aircraft. If you think you need to check for water after you fuel, I would not put that fuel in the plane at all! If you think you need to check for water because you fuelled, you have now missed your opportunity for some time, while the water separates back out of the fuel again. I'd wait half an hour.
I have owned my 150M for 26 years. Never ever have I drained more that about a CC of water from a tank sump. I have drained more from the belly low point, which as mentioned, comes capped, and must have a drain installed - I did. That said, the "gascolator" is down stream of that sump, and should catch water before it goes up to the engine. Of course, drain it preflight.
Know that water in the tanks can also come from condensation inside the tank, and thus have nothing to do with the caps. Leaving the tanks full will prevent this (and is good for bladder tanks).
On the other hand, a 182 I used to fly a lot always drained some water from the tank sumps, so I always checked it.