There's 2 ways -- one is to check the actual e-brake cable. Look on the driverside frame rail more towards the rear and follow the cable 'til it hits a bracket and splits into 2 cables. It can be adjusted here.
Also you can use a flathead screw driver on the insides of your drum brakes (no wheel removal required) and adjust the star wheel inside. This adjusts the shoes -- you'll probably want to jack up the truck and have someone spin the tires as you do this, and adjust it 'til it just grabs the wheel while it's spinning freely.
Our style truck doesnt have the adjuster where it splits. At least mine doesnt. My adjuster is in the e-brake pedal itself. There is a way to pop it then set it. I had to do that when I broke 1 of the cables.
Umm, are the self-adjusters gone on your rear brakes?
My '92 has self adjusting brakes, so I'm relatively sure the '93 had 'em too. If they ain't staying adjusted, chances are, the adjusters are rusted into one big blob, or the star wheel is siezed, and you won't be able to turn it anyway. And NO, the self adjusters do not mean your brakes won't last as long. 155K, and STILL has the factory shoes on the rears. P-brake holds just fine, just like it did the day I got the truck. In fact, the ONLY things I've done to this truck for brakes is replace the front pads, and flush the fluid at 140K. Even with some really nasty looking black ooze, the brakes worked perfectly.
If the self adjusters are working properly, all ya gotta do is back up, and hit the brakes firmly. Not enough to slide the tires, just a good quick stop in reverse. Each time you do that, the adjusters will turn the star-wheel a notch until the brakes are tight. Before you do anything to adjust them, I'd pull a wheel and check the shoes and drums. P-brakes don't just get loose. If the cable is stretching, then it's got broken strands.
One word of note, though, if it ain't holding good with the back end down hill, that's actually normal. The rear brakes are designed for better braking forward than backward. I have to set the brake a LOT harder with the nose up hill. Doesn't take much at all to hold it with the nose down hill. My old 73 E-350 decided to leave one night when I didn't get it in park good, and the p-brake didn't hold. Pug nose E-350's look really weird when the rear bumper is 6 feet below the front bumper.