This is a test of commonsense. Complete the next sentence:

How to turn an old road bike into a singlespeed 
Check the rear dropout shape. 
Does the bike have horizontal dropouts in the back? This is necessary to be able to adjust chain slack in the final step. Alternately, you can buy some kind of doohickey to do it, but that ruins the clean look so vitally important to most conversions.
What this means is, can you adjust where the back wheel goes, in a front-back direction. Some bikes have vertical dropouts, some have slanted ones.