ATTENTION: Orders placed between March 3-8 will be processed on March 9. Please check or our Dealers page if you wish to purchase during this time. Thank you for your understanding! - Robin

How to tell if you have a Small or Large bore Trombone

Easiest, but not 100% accurate:

If the student is playing on the same trombone which they started with, it will most likely be small bore, as most beginners won't start on a large bore.  

Generally Large Bore Trombones have an F-Attachment as picture below (left). Small bore (right).