Umm...maybe look at them this way: which ones have the burnt-on carbon? The intake side is usually quite clean.
Another question: are you installing new guides? Because if not, you're likely headed for some trouble, here. You can't pull the guides out and just reinstall them: the valves will no longer align with the valve seats and you must recut the seats to match the new angle of the guides. If not, they will not seal.
When the guides and seats were first installed, the faces of the seats were cut with a valve seat machine to ensure the valves seated and sealed. This is necessary because there is always a slight misalignment between the axis of the valve guide and the axis of the valve seat, due to the way they get installed. The final step in making the valve setups is to cut the seats with a tool that is piloted in the valve guide, after that guide has been honed to the final ID dimension. There is always a slight angle to the finished result, so whenever guides are replaced this process must be repeated. "Bad" shops would sometimes try to just increase the bore of the valve guide (i.e., make it looser) so a valve would seal at the seat after installing a new guide, but this just makes that valve leak (and burn) oil.
