I checked this before and never found a fuse that was not always hot, with or without the ignition.1. Open the hood and remove the lid to the fuse box.
2. Read the inside of the fusebox cover, it has a diagram of what the fuses are for. The diagram will also show you where the fuse puller is.
3. With the car off and key removed pick a fuse that would withstand the extra load introduced by your accessory (even though it has it's own fuse).
4. Test the fuse for power, if it has power, it's constantly on, avoid that one and move on to the next fuse. Continue procedure until you find a few fuses that do not have power in the off position.