Akiva, I played with a Nikon 50mm f1.2 on my D700 some time ago, and found that the focusing screen works as a huge focus indicator. In other words, if the entire viewfinder looks sharp, the image is probably in focus.
Now, the D700 (like most AF bodies) has an electronic rangefinder that gives you a green dot, visible on the bottom side (a bit to the left) of the VF, which indicates focus is correct. In case of doubt, you can trust it. Just leave the focusing point in the middle and use it as reference for your manual focusing.
Keep in mind that the D700 has a nice VF, so if you use fast glass, you'll be able to judge on your own.
You have very good friends who lend you neat gear. Show them your appreciationg and have fun with the camera! 🙂