How long does the "marked for death" shout last? Is there any way of telling when it has worn off?

How about the "muffle" spell?

I've got similar questions about other spells and shouts, but this will give me the general idea.

Thanks, Daddyorc

I am not sure about the duration for marked for death- but it is easy to tell if it is in effect or not. Things being effected by it will glow with purple "rune" patterns.

As for other spell durations- cast them, pull up magic-active effects, and see for yourself

According to the UESP wiki is it 60 seconds for all three versions.