But, how do I verify that it's working?
If you are using Continuous AF, I believe, AF-assist will not work. But don't quote me on that, it's been a while since I used F5. Also, see if you can access SB-28DX's menu and check if AF-assist is on. Not sure how to do this, never used that flash. I know on my SB-800 there is such option.
Does it still work with the flash in standby?
Pressing the shutter button half way down, or in your case pressing AF-on button, should "wake up" the flash from stand-by mode, if mounted on the hot shoe on top of the camera. If you are using any kind of remote triggering system this might not be the case so make sure you test it.
You can always refer to the manuals and see what it says for both flash and the camera. Or if all else fails, like Matt said, go into very dark room and play with settings.
Look trough the viewfinder if you can see vertical red lines when you activate your AF then it's working.