With a large tray/flat tub/container that “level” surface can become a serious problem. Nothin is level around here
I usually compensate by rotating so high and dry end is submerged, but this still means the pots in the middle tend to get less water. It’s a good idea to reorganize/rearrange once in a while — not Possible with single sheet connected flat of cells.
...in addition to heft-check, another -visual- check is to see if potting mix is pulling away from the side of the container (drying and contracting) and leaving a gap.
...also VERY helpful to use sand-grain sized diatomaceous earth — sold as autoshop oil absorber (Ultrasorb) — rinse Several times, then put on top of the mix. Tan/grey when wet, dries to nearly pure white when dry — great Visual indicator of surface dryness.
...I use turkey baster to apply small amounts of water to containers in the middle that needs it. Also good for draining overfull trays, but when in larger volume, it’s easier to use a tubing and siphon it — turkey baster again for 1/2” diameter tubing ... secure one end against bottom in the tray at deepest point, fill baster with water, push water in from other end and when you hear bubbling, suck the water back out, then trail the end down below tray in a bucket until all water is drained.
...I use a 1/4” air tubing and a plastic pipet for smaller amounts of water.