It should have a dark brown, almost crusty edge and be deep red in the middle. Most sushi places will season it with seasame seeds, salt and pepper, then put it in a blazing hot pan for 1 or 2 minutes per side.
Per Sage's comment - you don't need to worry about raw tuna for food safety - if you are getting nice tuna steaks you can simply slice it up and eat it raw if you like (which I do). Grilling works well too with Tuna, same thing, get the grill blazing hot, and sear both sides quickly, all done.
