How to tell choke size?

Most manufacturers should indicate the choke constriction somewhere on the barrel. It should at least be listed on the data information on the box if the gun you purchased came with the original box. It may not be spelled out but in some type of symbol such as Browning uses ** for Modified, or * for full, these can be found at the receiver end of the barrel.  A couple options you have are either contacting the gun manufacturer and speaking to their customer service, internet search for your particular make and model, or there are shotgun bore and choke gauges available for purchase. Ask around at your local club someone may have  a choke gauge that you can use.