I'm always leery when doctors o' philosophy ask simple, innocent, open-ended questions. A little alarm bell goes off in my head, warning that I may wind up on the short end of a Socratic dialogue.

Without the manufacturer's specs at hand, it's hard to determine an ideal load. A 20 Ah battery is not that large, and there's a fine line between a load that's large enough to test the battery strength and a load that will overheat/damage the battery.

The safest route is to use the on-board inverter as your load. It has built-in protective circuitry. Plug in an item that's near its maximum capacity, and then measure the DC voltage of the battery. If the voltage holds well, you have a good battery and charge; if it drops like a stone, you have either a weakened battery or incomplete charge.

Another option is to charge your batteries on the solar panel, let them rest a short while, then use the wall charger supplied with your unit. These usually kick off when a full charge is achieved.

As for field expedients, a DC motor (e.g., large bilge pump) would work. I think an old-style sealed automobile headlamp would be quite viable (IIRC, these pull somewhere around 5-6 amps). Connect it, measure the voltage, and disconnect.

A last caution: these batteries are not deep-discharge types. Discharging them too deeply causes damage. If you're using the inverter, it will alarm and shut down when the voltage becomes dangerously low. This is just under 11 volts IIRC. If you're using direct DC, there will not be an alarm or automatic shutdown; you have to stay on top of it.