Can be picked from the colour of the flesh like Mac tuna but you should be getting high end fish fillets . Very hard to differentiate between longtail tuna, bigeye, and even yellowfin without seeing the skin side of the fillet and even then big eye can be hard to pick without seeing the long sickles on a yellowfin.

 ...the easiest and most accurate way of telling these two apart IS through differences in the flesh. Most Yellowfin do not have the sickles, and are very difficult to externally separate from bigeye.

At one point or another in my life, I've caught, filleted, cooked, and eaten (or attempted to eat, lol) yft, sbt, albacore, and stripeys, but only albacore (white flesh) and stripeys (beetroot flesh) really stood out as being different.

 Thank you very much Striker and Auslobster.

Iooks like i need to research ... i mean alot