My 99 Ram 1500 5.2 won't start. I am thinking it might have a drain on it, but I have no ability to test for it. Is there any simple method to test it?

Why it might be the battery:
Battery is at least 4 years old. Actually, cannot remember when it was installed at all and cannot find a date on it.
Ran a trickle charger on it for several hours yesterday, it started yesterday, but ran very weakly.
This AM the battery is drained to the point the door chimes won't even light.

I was inclined to just replace the battery, until I saw the price of them.

UPDATE:

I was able to know it was a drain. I was not able to figure out what was draining it. I took it to a garage. While there it operated perfectly without draining at all. I got home from work and had a friend drive me to pick the truck up. I am figuring, it is going to be one of those really annoying intermittent problems and I am going to fear driving this thing. Anyway, I left the truck in the middle of the pull around jumped in my regular car to take my friend home. When I returned I pulled in behind the pickup and saw the backup lights were on. the lights on the top back of the bed were shining nice and bright. the way I normally park it I am not able to see the back of the truck.

Now I have to see if I can find out why. My first guess is the linkage on the shifter isn't hitting the switch. But I actually have no idea how it actually works.