[SOLVED] How would you figure out if a value is a integer?

I have come across multiple instances where I would need something like this.

For example, I have recently made a banning system where you can either input a player UserID or a player's username. I was wondering how you would be able to tell if that value (which would be a textbox) was a number or a string value.

This would be very helpful and I thank you in advance.