to tell the difference between r12 and 134a if no stickers are available is by the service line connectors, r12 sytems have very small screw type nipples known as shrader valves, whereas 134a have service vales which are quite large and need snap on connectors to charge system, the low side on 134a connecters is smaller than the high side one, hope this helps if you still unsure email me and i will send you fhotoes of the different types