Get yourself a solid lifter and some silly putty, or likes likes of it. Smer the putty in the intake valve relief. Try to keep it flat in the piston. Put the head on, no HG. I usually put in bolts in the outside corners, and two in the center to 25 ft/lbs. Put the rocker on ans set it up like you normally would. Rotate the motor over once or twice to make sure the intake opens and closes. Remove it all and look at the putty. Make a cut with a razor down the center of the valve relief. This will let you look at how close the valve comes to the piston. You want about .100 or so. Make sure you look all around the putty also. If the piston isn't fly for that size valve, it can run into problems on the side of the piston, not just the center.
there are two ways to check valve to piston clearance, the most common way, and longest used, is the way 10secgoal described. the other way is basically the same except you do away with the clay, and use a dial indicator to measure the exact amount of clearance at various points along the valve timing curve. usually starting at about 20 degrees btdc, and ending about 20 degrees after. a couple rules of thumb, you need at least .080" on the intake, and .100" on the exhaust. second, if your valve lift is at or below .500" and your intake valve is 1.94" or smaller, then you should have plenty of clearance between the valve and the piston. check to be sure though.
it also depends what block you are using. in '72+ years ford increased block height by nearly 0.2". You should be safe with low volume heads and moderate (<0.5") valve lift