You remove the valve chest cover (held with 2 screws, below the muffler) so you can see the gap between the tappets (cam followers) and the ends of the valve stems. Turn the engine to a little past top dead center on the compression stroke (if you are a purist, turn it until the piston has moved 1/4" down the cylinder), then insert feeler gauges into the gap between the tappets and valve stems. The thickest gauge you should be able to fit in without moving the valve should be 0.005-0.007" for the inlet, and 0.009-0.011" for exhaust. Obviously you do this after you lap the valves, not before, because the clearance will decrease due to lapping. If the clearance is less than the minimum specified, you remove the valve concerned and grind the flat on the end of its stem to achieve the middle of the specified range. It is important to end up with the end of the stem square to the cylindrical part of the stem.
Note that there is a good chance your exhaust valve is not seating at present, so until that is rectified you might get an artificially inflated number for the clearance.
