How do I check a players X value in a previous point in time?

For example, if at tick 100 the player's x value was 20, at tick 200 I want to be able to get that. Same thing for tick 101, 102, etc.

How would I do this?

Save these values to an array or dictionary. For example:

On every tick
 Dictionary Set key "Tick_"&tickcount to Player.x

Use a one-dimensional array: construct.net/en/make-games/manuals/construct-3/plugin-reference/array

Then, at each tick, add the player's x value to the array.

So, you will be able to retrieve the the player's x value at any position in the array.