Check the schematics of the shields to see if they use any of the same pins.
Even if they do use the same pins, this may not be a show-stopper. For example, I'm helping student put together a 3-shield stack. All 3 have 2 SPI devices, and one has an I2C device on it. The SPI pins (11-13) are shared on all three shields, with each shield's peripherals attached to a unique SS line.
There may be the issue of mechanical interference as well. For example, my home-brew ethernet shield can't have anything stacked on top of it, since it's too tall.
-j