You will want to pick an interval of time to regularly check your inventory by visual inspection and a physical count. This will depend on how fast your inventory moves.
You will then want to acquire a list of any and all new inventory purchases for that time period (say, one week), as well as a list of the sales that occurred during the week (which represents inventory leaving).
You will then take your initial inventory count sheet, and do a full count of all your inventory. Your current inventory should be equal to whatever the count last week was, minus all sales for the week, plus the new inventory purchases.