On the Xbox 360 you used to press left on the D-pad, but for the PC I've mashed everything and nothing seems to detonate them.
I also don't see anything in the controls menu that makes sense.
By default the key is G. You can change it in the key bindings menu:
You can also shoot them if you get desperate. Of course, that will only detonate one at a time, unless they're on the same object.