What switch can be pressed from both directions?

[GeeTwo]

Originally Posted by EricH

You could also use a beam-break sensor.

If you are trying to do this with something much smaller than a tote (like a tab of angle aluminum), you can get an integrated system. Sparkfun also sells the breakout board. For about $4 you can have a sensor that's ready for a PWM-style cable that can plug directly into a DIO port. I just ordered ten of these yesterday, so I had the URL handy:
https://www.sparkfun.com/products/9299
We used one of these (or something very similar) and a plastic disk with holes as an encoder on our first robot for Rebound Rumble.