which variable can be used to judge?

I want to measure the movement distance(no more than 30 cm in single axis), now I use mpu6050. the toy move to left, then move to right, arrived the right terminal go left , arrived the left terminal go right, loops. I want to get the exact time when the toy arrive the left or right terminal and change direction.I got the pitch ,roll ,yaw and quaternions. I don't what can i do for the next step.
I wonder which variable can be used to serve as the event occurs? thank you very much!