Since the
oil pan is off and the plug told you what cylinder to look at, do as Lineslinger said "rotate engine slow by hand with plugs out. Watch the suspect cylinder's piston. At top dead center (or bottom dead center) rotate the crank back and forth. See how far you have to move the crank before the piston starts to move. Compare to some other pistons. If there is any play in a rod or wrist pin it will likely show up with this test. If your eyes are going (like mine), put a dial indicator on the piston.