Smell and feel are the most apt indicators of bona fide, sushi-grade tuna. Labels with "sushi-grade" or other wording indicating the tuna is safe to eat uncooked don't carry much weight, as they're not regulated by the U.S. Department of Agriculture. A rosy pink color doesn't indicate freshness, either, unless you're buying the tuna right after it's caught. Tuna starts oxidizing about an hour after it leaves the water and turns brown; purveyors treat tuna during processing so it retains a pink color longer than usual. You have to watch the tuna being cut and ask questions to know if it's fresh.
If the tuna lies behind a display counter, inquire if the market staffers cut the pieces off a fillet themselves or they purchased them already filleted. If they don't cut the tuna themselves, don't use it for sushi. Watch the fishmonger or seafood attendant when he slices the tuna off the fillet and look at the cutting board. The board should be tan, which indicates it's only used for seafood, or white, the color code for general use. If you see the tuna on anything except a white or tan board, don't buy it. Check the attendant's knife visually, if possible, before he goes to work on your tuna. It should be clean or, ideally, he dipped it in a sanitizing solution in front of you. Lastly, he should wear food-handler gloves while working with your tuna.
Check the tuna for freshness before purchasing it. Smell the tuna; it should have a clean, oceanic aroma with no trace of ammonia or the telltale fishy smell that indicates degrading proteins. Press the flesh with your finger; it should press back, spring-like. The tuna should have no discoloration or blemishing.