In snapshot 13w38a, the F3 status doesn't really tell you if a shader is off (the status stops changing once you get to the default), but it does in 13w38b - the line describing the active shader file diisappears. Apart from hitting F3 to see the details, I can't really think of another way to tell.
Some of the shaders are fun if impractical - the greenscreen shader is very nice (and looks better in game than from the static screenshot) but in a dimly lit cave you are in almost complete darkness. Plus the extreme motion blur shader might give some people headaches. The black wireframe shader looks impressive stylistically, especially on the ore blocks and some of the monsters, but the effect is somewhat diminished on highly detailed objects such as trees which look almost totally filled in.
Hopefully once the final 1.7 is released, the super secret settings might expand and allow greater control of selecting graphics profiles and even let us create our own, Winamp style.
EDIT: As of 13w38c, press F4 to stop the current shader and reset the graphics to normal. Also, it seems the shaders were put in to test future gameplay effects, perhaps adverse effects that the player can suffer under certain conditions? (Some of these, such as flip mode and extreme blur, I can definitely understand.)