If your travels take you past one of America's Army installations, you can't just hop on base and look around without permission. The process to obtain a visitor's pass depends on whether you have a military traveling companion and security threat level and procedures at the base.
With a Military Escort
If you're entering a base with a member of the military who has his Defense Department identification, the guard at the gate will ask to see a valid photo identification such as a driver's license. If the military member is a passenger in your car, be ready to present the documentation necessary to operate a car on base: registration and proof of insurance in addition to your driver's license.
Without a Military Escort
Stop first at the visitor's center, which at most military bases is conveniently situated near the main gate. You'll be asked to provide your reason for visiting, any documentation for your trip. You may be asked to register your car for base access. If you attempt to visit a base when the visitor's center is closed, or a base without a center, go to the main gate.
According to Military.com, in August 2014 the Defense Department sped up implementation of a background check system with the intention of expanding it to more bases over the next several years. The Identity Matching Engine for Security and Analysis scans DoD personnel with passes as well as visitors through FBI databases, meaning you may be asked to present a Social Security number and date of birth when requesting base access.