I was taught at the S&W factory revolver school that you slowly thumb cock the empty revolver. With the hammer back in the cocked position you give the cylinder an nudge and if you hear the cylinder stop click into position the revolver suffers from DCU or doesn't carry up. It's been a few years but a revolver with DCU on any cylinder notch was considered defective and repair was in order. The revolver was not designed to require speedy manipulation to lock up correctly. DCU usually gets worse over time and is fairly easy to repair.

I usually pass on a revolver with DCU unless priced low enough to repair after purchase.