How do I tell if I have a Nashville or ABR-1 bridge on my Gibson® guitar?

Basically, without first discussing the bridge itself, the first clue is the type of height adjustment hardware you have. ABR-1 systems have threads along the entire length of the bridge post, with a separate thumbwheel which rides up and down the threads to adjust the bridge height. The bridge stud is mounted directly into the wood. Nashville systems have a combination one piece post and thumbwheel which screws into a threaded metal insert in the body. The combo bridge post/thumbwheel is threaded on the bottom only, and the bridge height adjustment is determined by how far the post/thumbwheel is screwed into the insert. There are no threads above the thumbwheel.
For someone familiar with both ABR-1 and Nashville Bridge System bridges, just seeing the bridge is all that is necessary.