I built a quick and cheap one, using a spare plastic case and some leads from a dead boost pack. The unit uses a cheap panel mount meter, showing battery voltage, and the test load is applied via a push button switch. A chunk of old heater element, to give around a 10A load to the SLA battery, so you put it on, look at the voltage, and press the switch for 10 seconds, or till voltage drops below 10V, whichever comes first. If it times out without dropping below 10V it is still useful, but generally if it dies in 5 seconds it is dead, and if the battery rattles not even worth bothering.
Did however test a spare angel eye lamp set, leaving it on a dying 18Ah battery for the last 5 days, and today it finally is barely lit, just lighting up all the dies still, but current is very low, and it barely provides light. Supposed to be a 320mA current per unit, but now probably under 10mA, though not going to put a meter in. Want to see if it will do a week plus, though the second lamp has a little leakage on the one segment of 16 LED dies, as those dropped out this morning, leaving the other 2 in series still lit. now only one left on it.