When you have an even number of rounds left in the magazine the top bullet is on the right side of the mag, an uneven number the top bullet is on the left. After load a mag an than you c*ck your rifle remove the mag an see which one the top bullet is. If the top bullet is on the left side you know you chambered a round.
You can also pull back the cocking handle slightly an look into the chamber.
If you ever get a chance to do the PWT4 Gunfighter program you will be taught this an many other drills.