Just to be clear, the folding stock is probably a ruger stock. The gas block is separate and people don't swap the gas block when they swap the stock. The sling mount is on the left side on the folder, so it should be on the left side of the gas block on stuff that left the factory but on most rifles where the stock was swapped, the sling mount is under the gas block, which is in line with where it is on a fixed stock rifle. That's one preliminary way of determining a stock swap.