Add up the wattage of the appliances you turn on.. when you reach half of your generator's output your there. I use a 10' extention cord and go from appliance to appliance, testing each one, like the toaster, blender, coffee maker, fans, heaters, etc. It's better to know now if they work or not than in the middle of a trip and it puts a load on the Genny. Besides, you have to do something during that half hour.
ps. also check the water in your batteries, just because your already there.
Here is a link to a good list from Good Sam ERS..