If you still have the box that the tool came in it should be marked on the box or packaging.

On some staple guns, the type of staples that fit might be etched or marked on the tool itself. 

If you know the model you have you can use the Rapid staple guide to find the staple you need for your staple gun.

See our staple guide to find correct staple, regardless of brand.

Read our guide on the different types of staples available.

To determine which staple gun you should purchase you first need to look at what you are fastening. The material you are fastening should determine what staple you use. Knowing what staple you need determines what staple gun you should purchase.

Read our staple guide to determine which kind staple suits the material you are fastening.

After that, you can read our guide on finding the right staple gun.

Thickness and type of material determine the length of the staple that should be used. 
The standard rule is tha the leg length should be three times the thickness of the material you are fastening.

Read more about leg length here (scroll down a bit on the page).