OP you should check your lens. Focusing at infinity might work, but my lens is a hair off. If I can I focus on the moon because I've found that if I just set to infinity on my lens and body I'm a tad off tack sharp. I'll use the moon, or shine a powerful headlamp on a tree more than 30' away and focus on that. Another thing I've used are city lights off in the distance. You'll need to experiment with your gear to see what method will work best.
The problem with the infinity bit is unless you're using a prime, there is no 'fixed' infinity. It's different depending on your zoom range. I have my autofocus set to only be active when I push the * button with my right thumb. It keeps me from having to fumble with the AF/MF switch while wearing gloves.
Just remember to recheck your focus anytime you alter your zoom.