You need to know the depth of the tank (fairly easy to determine) and the height of the outflow (not so easy to determine, unless you can look in the tank).
The exit is usually a T (lying sideways) or an el, pointed down. You want the bottom layer at least a foot below the outlet pipe and you want the floating layer thin enough so that it won't enter the top of the tee or the el.
You can measure the depth of the top layer and the height of the bottom layer by feel. The stick will have some resistance as you push it down and then no resistance as it goes past the bottom of the floating layer. The bottom layer with have resistance and then the pole will stop at the bottom of the tank. To keep track of the depths make chalk marks on a second pole held on the ground to indicate the top of the floating layer, bottom of the floating layer, top of the bottom slush, bottom of the tank.
As a general rule you probably shouldn't have a bottom slush of more than 2 to 2-1/2 feet and the floating layers shouldn't be more than 1 foot thick for a residence size tank.