Hey Paul,
I run the gpx 4800. One method I use to determine whether the target is trash or not is to toggle between enhance and normal. Typically(not always) when in enhance most targets will give me the low/high response (woo/wee). When toggled to normal the response will either stay the same, most of the time indicating junk...key word being most(not always). Or the signal will sometimes switch to the high/low response (wee/woo) which always gets me excited because that most of the time indicates non ferrous.
Some of the variables that I have found to eliminate this advantage is the depth and size of target and type of ground you are running in.
Best not to use this as the end all of your to dig or not decision but often times it helps. Pay attention to the strength of the signal and what response your detector is giving and you can get a lot of info about what is down below. Try testing this method it helps me a bunch.