Find an overhead light source (ceiling fan light, the sun etc). Wear rubber gloves. Hold each lens by the edges, grooves facing the ground. Lift the lens from the ground until it focuses to the smallest point it will make. Measure the distance to the ground. The lens that focuses closest to the ground is the back lens (220mm collimator).

I've added (and edited) The Wiki Guide to have a section on how to do this.

In short, stand up the lenses, and place something behind them. I put them groove side out, but I've tried it both ways, and it still works. I used coke cans the first time, and Bawls bottles here:



Now go around front and look at your lenses.



For placement in the projector, the grooves face each other. I just remember that they need to be protected, so they aren't exposed to the outside.