I've always found flipping a wheel in the stand to be pretty useless for determining dish. This goes for both shop grade Park stands, and any kind of home economy stand. They are too wiggly. At most you get a rough notion of dish. Using the bike itself and zip ties or the like works better.
An actual tool is nice to have, but if you're not working in a shop, you can work around it. In my LBS days, I most often used the campy dishing tool. VAR made a nice tool. These days I have the cheap Minoura tool. Good enough for me and my own bikes. A DIY dishing tool would be pretty easy to fabricate if you build a lot of wheels.