Evan,
You wouldn't need to build a nice stand like Hilditch has but when weighing the tongue it must be at the height it would be when hooked to the car. This could be done with some blocks and scrap wood piled on the scale and zeroed before placing the tongue on it. Notice I didn't say it should be with the trailer level, because rarely is the trailer perfectly level when hooked to a car. Try to match the hitch height. If you just set the nose of the trailer on the scale it will show a super high tongue weight.
Barry and Annie,
I would think you wouldn't need to be checking this while traveling. This is something I would do at home in the garage when loading the trailer so you have a good "feel" for how items you place in the trailer affect the tongue weight. Doing it this way will make you very comfortable with your loading while traveling.
Hilditch pointed out a very important point. Don't let the tongue weight get below 10% of the trailer's total. This is for all trailers. It will qet a bit squirrelly. We build the 550s with approximately a 12.5% tongue weight (depending on options, some add more). This is to give you wiggle room on your loading. The 550s are hard to get out of whack loading, as you load it the tongue gets a bit heavier. The 560s have a heavier starting tongue weight (about 14%). This is because of the trailer being longer and having more moment arm behind the axle. So, as you load the trailer the heavier stuff tends to be in the galley area and it reduces the tongue weight rather than adding it. So, the tongue weight percentage will typically get lighter on a 560 as you load it, even Evans.
Cary