Ahhh...there is an easy way to check if you have an anti-siphon in, but if you're going to upgrade to an On/Off it's easier to just make sure your On/Off has the tube installed.
Most bottles that have an anti-siphon tube, have a mark someone on the valve body or bottle neck, indicating either where the tube mouth is pointed, or indicating which part of the bottle should be facing up. You simply get a full bottle (don't overfill) and a loose ASA (one with a built-in On/Off is nice) and start the gas flowing while you wear gloves and hold the bottle horizontal and you quickly, but steadily rotate the bottle along it's long axis which should be parallel to the ground. If there is one point where the white gas cloud increases as the bottle is rotated, then that point is where the anti-siphon tube mouth is pointed, and you want to make sure that point is up when you screw the bottle into your gun. It's easier to do than to explain.
Pointers:
-Don't mount an anti-siphoned bottle into a vertical ASA unless your gun likes liquid CO2.
-Don't use an Anti-siphon bottle with a remote setup unless your bottle is horizontal in your harness.
Ray