a lot of pausing, repetition, and retests. I don't feel like explaining it, but just really boring method of standing 2 characters next to each other, pausing, then holding A on both controllers (or whatever is needed) and seeing if the 2 moves collide at the same time, then it's the same speed (lots of retests to assure accuracy). And if it's one frame apart, for 2 moves of close, but not exact speed, (I can do up to a few frames if I wanna be really tricky but that's really frustrating to test) I just press A on one controller, then pause immediately, and on the other controller I hold A, and then unpause, and they will collide on the same time. Lots of retests.
However, Brawl made it really hard for me since I can no longer hold a C stick direction with pause. In melee if I did that I'll just smash attack as soon as possible (when it's unpaused), as expected, but in Brawl it doesn't let you do that for some STUPID reason, making these tests way more boring.
Btw, for a comparison, Snake's Jab is 3, F tilt is 4, Up tilt is 6 and Down tilt is 6