When trying to detect aromas, it helps to know that each grape varietal has a few classic aromas. Here is a guilt-free cheat sheet that covers several of the more popular red and white varietals. (Note that aromas are also determined by where the grapes are grown and how they are handled in the cellar.)
Cabernet Sauvignon: black currant, mint, plum, eucalyptus, bell pepper, olives, vanilla, black cherry, cedar, anise, cassis
Merlot: red berries, black berries, eucalyptus, mint, herb, bell pepper, plum, violets, cassis, fruit cake, chocolate
Pinot Noir: red currant, strawberry, cherry (red or black), raspberry, violets, mushrooms, decaying leaves, cola
Syrah/Shiraz: raspberry, black or white pepper, blackberry, red or black currant, cassis, jam, smoke, leather, tar, coffee
Zinfandel: Wild berries, raspberry, plum, pepper, bramble, earth
Chardonnay: apple, melon, peach, pineapple, pear, lemon, fig, honey, butter, toast
Sauvignon Blanc: grass, gooseberry, nettles, herbs, tropical fruit, citrus, fig, cat pee
Riesling: green apple, lime, peach, grapefruit, honeysuckle, mineral, slate, floral, petrol, toast