If it’s not a drain plug, then just take a hose if you can and put some water into the livewell to find out how it works. It should be pretty simple to figure out.
You can take a look at the livewell pump to see how it’s plumbed, but the livewell will probably drain out back through the pump when it’s out of the water. There could possibly be a valve inline to turn off flow through the drain/fill hose near the pump also, but you’ll see it when you look at the pump.
If it doesn’t drain when you think it should, there could be dead minnows (or something else) blocking the drain/fill hose too. If you think it may be plugged, just take a hose and hold it up to the intake hole at the back of the boat and force water through the system to flush out the drain/fill hose into the livewell. If the water goes in it should come out.