You never mentioned what's on top, a complete review of other possible HVAC runs, and 
other piping might need to be considered. Ex. most modern house use rafters for branch runs, while old houses are feed from the bottom, if with crawl space. So ID'n everything in a this space and is IMO a wise application.
Buy a stud finder, while the client has some thought of the install, you don't want to 
chew up timber to get it on the spot.
  
If your so inclinded; simple graph paper (which will help creat a scale) and stud
 lay-out imposed with clear this up.  You can also use opaque wrapping paper as an
 over layer to light patten to a scaled kitchen.  BTW you can have multi sheets layed
 over each other.  I bought onion paper (roll) which does the same thing, but it's pricie.
Blue painters tape is also a handy applicator.