I learned these techniques while attending Thunder Ranch. It is an excellent school and I would recommend it to anyone interested.

In a gunfight, you may find yourself in a "lull" when the action has stopped. It may began again, without notice. In the mean time, you would want to do a "Tactical Reload".
This means to replace any partially loaded magazines with full ones. You do this while keeping the weapon pointed towards the threat and ready to go into action immediately.

As you pull out the new mag, you use your finger to feel the top round and make sure it is properly seated in the mag.

You would then remove the partially loaded mag and switch to the fully loaded one.

Do not drop the partially loaded one.

As Clint Smith says, "There may come a time when you will be really, really glad to have those extra few rounds."

You should then give the new magazine a good bump with the palm of your hand to make sure it is fully seated.

Place the partially loaded mag in your pocket.

You are then faced with a question: "Is there a round in the chamber?"

To confirm this, you must do what is called a "Systems Check".

You slightly pull the slide back enough to place your finger in the open chamber and feel for the loaded round.

You "feel" rather than "look", so that it can be done in low-light situations as well.

Be sure to keep your off hand clear of the muzzle at all times.

After feeling the round, close the slide and return to firing position.

You now have a fully loaded pistol and are now ready to go back to work.