GameObject Y movement speed

Hi there! I have an FBX character, and a controller script on it. Only thing what i trying to achieve is checking the character's sudden Y directed speed. So if my character started to fall, i want to change the animation and pose to falling animation. But i don't want to do the same animation, when my character is going down on a slope. I tried so many script, but none of them worked for me. I don't want to use Controller velocity, and if possible, i would be happy if there would be a solution in JS. :) Thanks!