As everyone has stated thus far, it is a “feel” thing and your body needs to learn what that particular feel is like. With regular trips to the range, you will know what your weapon feels like when it cycles and feeds another round. Recoil, slide comes back, trigger is reset, spent round ejects, new round comes up, slide goes forward feeding new round, slide seats fully forward and your weapon is back in battery. It is something you feel.
When your slick locks back, it will be a completely different feel because the cycle stops halfway through.
PRACTICE, PRACTICE, PRACTICE.
Another thing to think about after the magazine is replaced is how you manipulate the controls of your weapon to get the slide back forward. Silly question, I know, BUT…
There are two different ways, both very common, to accomplish this. The safe way, as is taught by many instructors across the world, is to grip the slide with your off hand, pull it back, release the slide lock with the thumb of your strong hand and then release your grip on the slide with your off hand and let the slide go forward. It is safe. Decreases wear and tear on the weapon, reduces probability for malfunction, etc, etc…
Or, if you are like me with my weapon, you can just hit the slide lock and keep shooting. It is quicker. Add the fact my weapon, a Beretta 92A1, was designed to be able to just drop the slide lock. It does not add any additional wear and tear to my weapon and I have NEVER experienced any malfunctions doing it this way. I have, however, been chastised by nearly every instructor I have been in class with because it just isn’t right. They teach it one way to fit everyone, then there are the wildcard Beretta Owners.
Pick your poison carefully.