Nice catch Gilly!!! Your on them now!!!! You DO have something to check pan tension with…..You just don't know it. However, it appears that you have them pretty close since you are catching. If you have been getting deep, or above the pad catches, then you are close on the tension. If you get toe catches, or are missing, then they are too light. If a rabbit or squirrel sets them off….too light.

Two methods, one long, one short. Take a 2x4, and cut it to length to weight a tad over 3 pound. A typical dry pine 2x4 will weigh approximately 1.6lb/foot. So cut one 2ft. 4" and that should be close. If you have a scales, actually weigh it.
Long method. tighten the pan tension screw and set the trap. LIGHTLY set the back edge of the board on the center of the pan. If it does not go off, lighten the the screw a little at a time till it does.

Short method/AKA scare the chit out of you method.
Tighten the pan enough you know it will hold the board. Might have to have someone steady it. Then, reaching under the jaw (its tricky), start loosening the screw until it goes off. make sure you keep the tools UNDER the jaw!!! It really helps to put the base of the trap in a bench vise. Tighten it down on the base plate really tight, and the trap will not jump.