As defined by Wikipedia, "A cocktail is a style of mixed drink... A cocktail today usually contains one or more types of liquor and one or more mixers, such as bitters, fruit juice, fruit, soda, ice, sugar, honey, milk, cream, or herbs."
There are thousands upon thousands of different cocktails out there. Granted, many of them are fairly similar, but you're still left with a considerable number. Since this topic isn't going to divulge 5,000 or so drinks to be manly or not, here are some guidelines.
1) Color: This is your easiest indication of a drinks manliness. If the bartender adds food coloring, usually red/pink, your drink is going to be farther from manliness than a male contestant on American Idol
2) Garnish: Most chick drinks have a citrus garnish on the rim.
3) Ingredients: There's a lot of overlap in this, so you really can't tell by a sole ingredient. Most girly drinks will contain either vodka, Southern Comfort, or rum. Drinks considered manly may contain gin, bourbon/scotch/cognac, or vodka.
4) Name: If it sounds gay, it probably is and if you can't infer from the name that the drink in question is a beverage, it's not manly.