Hardware Universe images and LED views will usually tell you which is port “a”.
PCIe Adapter cards always have the convention that the top port is the first or “a” port. “Top” is the port furthest away from the PCIe connectors (which is the “bottom”). When onsite, if a card is horizontal, the connectors will tell you which side (left or right) is “bottom”. Generally, for horizontal PCIe card slots, the riser is on the left, which means connectors are on the left. So, the “a” port is furthest to the right, “b” is to the left and so on.