There are a few ways to determine which choke tube you have. The easiest way is to look at the end of the choke tube. If there is a raised letter or number on the end of the tube, then it is a fixed choke tube. If there is no letter or number on the end of the choke tube, then it is a variable choke tube.
Another way to determine which choke tube you have is by looking at the size of the opening at the end of the choke tube. Fixed choke tubes will have an opening that is either .035″ or .038″. Variable choke tubes will have an opening that ranges in size from .027″ to .032″.
You can also determine the type of choke tube you have by looking at the markings on the barrel of your shotgun. shotguns with fixed choke tubes will have the constriction size (e.g. “Mod” or “Full”) stamped on the barrel near the muzzle. Shotguns with screw-in choke tubes will have markings that indicate which type of tube can be used with the gun (e.g. “Invector” or “Remchoke”).