Another good option somewhat like ewarnerusa's is to permanently install a little panel voltmeter and ammeter (or combination meter) to measure the battery voltage and the charge/discharge current. Those together can tell you most everything you need to know about what's going on with your battery.
Something like
this combo meter should do the trick nicely. (Note: I have no experience with this particular meter--but it looks like it should serve.) Look for one that can measure current in both directions; some only work one way, which means you can only measure charging or discharging current, but not both (at least not without an additional switch to swap the shunt connections around).
One advantage of a permanently mounted meter is that you can keep tabs on what's happening without having to muck around in the battery compartment. It's a lot more convenient for monitoring, but nowhere near as versatile for troubleshooting. Ideally you might want both a multimeter and a panel meter.