You need to post some pics of the skimmer housings and to look at the bottom of the skimmer housing below the baskets to see if there is two pipes coming in.
On ours, there is a low water level float valve that sits in the very bottom of the skimmer housing below the basket that has to be lifted out to allow you to see the piping. The piping in the bottom has 1 opening that runs to the pump and 1 that goes to the bottom drain in the pool. The float valve has a movable plate that allows you to adjust how much water from the skimmer vs. bottom drain is pulled by the pump. If the skimmer goes dry, the float drops and seals the basket so all water comes from main drain and pump doesnt run dry. Yours may be different but,generally if 2 pipes are in the skimmer housing bottom, 1 is likely the main drain.
BTW you dont want to have any noticable suction at the main drain due to entrapment risks. Your pool is likely old enough that a safety drain wasnt required so be careful!