Start with the name printed on the pen body, battery cover or original box. Common Dr. Pen labels include A6s, M8, X5 and similar series — write the exact characters you see, including letters after the number.
If the print has worn off, photograph the full pen from the side and the cartridge bay with the tip removed. The bay shape and drive pin layout are often enough for us to narrow the model.
Do not rely on colour or overall size alone. Two pens can look similar and still take different connectors. When unsure, send photos through a sourcing request before you add items to an order.