A remote wire will test with around 12 volts whenever the head unit is switched on.
It's only found as a blue/white wire on after-market head units. On a factory radio, it will be a different color, if it exists at all. Most factory head units won't have a remote output unless there's a factory amplifier in the system; and even then many use a different method of activating the amplifier.
If you don't have a true remote wire, you can connect the remote connection from your amp to the accessory power wire. This will test with 12 volts whenever the key is turned on. It's much more common than remote wires in factory audio systems, and works almost as well.
If you're trying to install an amp with a factory radio, then edit your question with the year and model of your car, and we might be able to provide more specific information for you.