Start with the simple stuff. Ground first -- check the resistance from the negative battery terminal to the engine. There should be low resistance (2 ohm or less). If it's higher, then look for a corroded terminal or ground lug.
If the battery is a lead acid (most are, the Optima and a few others are not) and it's more than a couple years old, it's time to have it tested. Any parts store should be able to help you out with a load test. (But on the off chance that you walk out and it doesn't start, check the voltage across the terminals. It should be at 12 volts.)
If the battery checks out, start the engine, and voltage across the battery terminals should be 14.4 volts or reasonably close. If not, have the alternator tested. This should also be included in a parts store battery load test.
It sounds like you may have already ruled out the battery and alternator, so you can probably continue from here.
Shut off the engine, take out the key, and disconnect the positive battery terminal. Now check for resistance (horseshoes on the meter) with one lead on the negative battery terminal and the other lead on the positive battery cable clamp. You should have an open indication (high ohm reading). If you do have resistance (low ohm reading), start disconnecting what you can, one fuse at a time. Feel free to plug it back in if there's no discernable change in resistance indication.
If you pull all the fuses and nothing changes the resistance indication, then it's got to be something hardwired. Keep the meter connected as before, and disconnect each accessory one at a time. Again, feel free to plug it back in if there's no change.
Take it one thing at a time, and the cause will pop up eventually. Patience is a virtue.