How to check torque converter?

That really doesn't do anything to test the converter operation. It can somewhat tell you the stall speed of it and if the transmission is starting to slip but because you are not in motion it only tests the lower clutches while at the same time the transmission is at a higher pressure. What it does test are the brakes, if the car moves the brakes are not holding. The other thing that will do is heat up the transmission fluid faster and that might let the transmission work better if it has worn seals and rings as the heated fluid can make those parts soften slightly and possibly seal better. This "test" is also only really valid on a hydraulicly controlled transmission, in modern electronically controlled units they are programmed based on the vehicles design and options and by the driver's usage. So, you might jump into a car and think the transmission is slipping, but grandma who drives it rarely goes over 55 and never hard on the throttle so it has "learned" to slip some between gears for more comfort.
They also learn much of this from YouTube, where the reality is folks stand on the brakes and hit the gas to be able to launch the car like a dragster.