we always use t his option to check - especially when files are to be printed
in black and white. We book ad space for one client of ours who has a
"designer" prepare his artwork. The designer prepares it in InDy (I think) and
supplies us with a pdf. Every time we check it the "black and white" artwork is
actually in CMYK with all four colours in the mix. We then have to open the pdf
in PS, convert to grayscale and re run the pdf (which we usually do through FH
using the method outlined above).

We then recheck and send off to the media.

Yes - we have told them time and time again - but they just don't seem to
(want to) understand....

ho hum

this method will also help you catch spot colours if some have slipped through
on a job which is supposed to be CMYK, or, in fact, help you see that what was
supposed to be a spot colour has been converted to CMYK (more likely) - all
useful