How do you know if your band has slipped?

How do you know? You can see your Surgeon!
He will do an “Upper GI X-Ray” that will show if the band has moved from the usual 45 degree angle, just below the Gastro-esophageal area...
If that’s the case, you have a option of removing the band. 
At times, if the slippage is minimal, deflating the system may help return the Band to the normal position.

The best way to know if your band has slipped is to get X rays done.  Symptoms do help make the diagnosis, but it still needs to be confirmed with x rays.  Consult with a bariatric surgeon so the he or she can order x rays that will help make the diagnosis.  Your surgeon may decide that an endoscopy may be the best first test.  An endoscopy will also contribute a lot of information.

although there are clinical and symptoms that can suggest if a band has slipped, the best thing to do is to actually evaluate the band; the two most common ways to evaluate the band are by x-ray, such as a 'barium swallow' or by endoscopy (EGD).
first and immediate step though, if there are any symptoms, are to deflate the band