For any workout lasting an hour or less, you do not need a sports drink; just water will do. Save sports drinks for after intense, continuous workouts that last an hour or longer (the key word being continuous). Sports drinks are designed to deliver carbohydrates, electrolytes, and fluids back to the body, which is essential after serious sweat sessions. During shorter workouts, your body taps into its ample stores of carbs and electrolytes, so sipping on water is enough to keep you hydrated.
By the time you're thirsty, you've already lost about 1% of your body weight in water, and by the time you have lost 2% of your body weight, you are entering the dehydration zone where performance is compromised. (For reference, if a 200-pound man lost 1% of his body weight in water, he'd have lost 2 pounds over the course of a workout.)
Caffeine has not been found to be dehydrating when consumed in moderation (1-3 mg/kg body weight, or approximately 2 cups of coffee for a 200-pound guy). Plus, there are several studies that suggest that caffeine improves performance in high-intensity and endurance exercise. The only drink that has a dehydrating impact on the body is alcohol.
Monitoring your urine is a great low-tech way to monitor your hydration status. The goal is to have pale lemonade-color urine. When your pee is completely clear, you are likely getting more fluid than you need. If your urine looks more like apple juice, that is an indication that you need to drink more.
Foods contribute to hydration, too. In fact, about 80% of daily hydration needs come from fluids and the remaining 20% come from foods, especially those that are high in water, like milk, yogurt, watermelon, cucumbers, and so on.