The simplest way:
Get a voltmeter and a solar panel. Connect the panel to your battery and watch the voltage rise. When it gets near 14 volts your battery is charged. Disconnect your solar panel to keep it from overcharging your battery.
Your battery can be damaged if you charge it too fast, so don't get a panel that's too big for your battery. Consult the battery manufacturer's data to see how many amps it can handle, both while charging and discharging. If you happen to have a big panel and a little battery, there are tricks further along in this instructable.
Some solar panels are made just for the purpose of maintaining batteries in vehicles that are parked a long time. They don't go over 13.5 or 14 volts and don't ever produce enough current to damage the battery. These you can just hook to your battery and forget.
If you want to use more power you'll want to get a bigger, higher voltage panel than that.
Here I am running my truck's electrical system off a solar panel wedged in the windshield.
My alternater fell off the engine and the belt broke. I'm driving south so I just set up the panel and drove into the sun, peeking over the panel. Read more of that Haywire Mechanic Story or click on "step two" to read more battery/solar tricks.
Step 1: Voltmeter Human Supervision
If the voltage suddenly jumps to 17volts I suspect the jumper cables that carry the solar panel current to the battery have gotten disconnected.
The panel output voltage rises to 17 volts or so when it's not connected to a load. When current flows the voltage drops due to internal resistance in the panel.
If your battery voltage gets to 14 volts or if you hear a bubbling sound when you put your ear to your battery it means you're overcharging your battery. Disconnect your panel. You'll get to know your panel, battery, and loads, and maybe you'll disconnect at a lower voltage, like 13.5 volts.
This method works fine when camping and you can lay around watching to see how your battery is doing, or if your panels can't keep up with demand so you don't have to worry about overcharging.