If you read a checklist,do you need a confirmation of the guy next to you that every item is done or do you just verify yourself?
The after take off checklist in the Boeing 737 is read aloud (challenge) by the PNF and he also replies (responds) to his own words. Boeing have used this philosophy since the first 737 flew. Despite this there have been countless incidents where the PNF has omitted to check that the aircraft is indeed pressurising and this has invariably been due to pressurisation switches mis-selection. The Helios Airlines B737 accident is an example.
Over hundreds of hours of simulator instruction, time and again I have watched the after take off checklist in the 737 ballsed up with the PNF reading and replying to himself without actually checking the pressurisation instruments and the captain too engrossed in other things to double check the F/O has done his job correctly.
Usually the first indication that the aircraft was never pressurising after take off in the first place, is the cabin altitude warning sounding at 10,000 ft followed by a startled "what the hell is that" from both crew members.
The whole point behind checklist philosophy is one person challenges and the other responds. Not so with the potentially deadly after take off checklist Boeing philosophy, where the captain is left out of the loop because he doesn't have to reply.