There are a number of methods that can be used to put a graphic like that on garments. Your example is an extremely simple design in three colors, very easy to print.
Without actually seeing and handling the shirt there is no certain way to tell what method was used to print the shirt.
If you are looking into how you might go about printing similar shirts yourself, here is a great thread to read through about all the different garment printing methods.
What technique is used to create this? 
Re: What technique is used to create this? 
