You can check the wheel rim is centred by using a dishing stick. Take the wheel off the bike and remove the skewer, and press the index tube against the lock-nut and adjust the stick until either end rests on the wheel rim. Lock the index tube in position.
Check the other side without resetting the stick. If the same setting works on both sides the rim is centred.
Using The Bike
Check the position of the wheel rim. Using a rim brake or an improvised index(link). Then remove the wheel.
Put the wheel in, the other way round. If the wheel rim is in the same place as before then the rim is centred.
In A Jig
A jig will have indicators to show where the wheel rim is. If you’re not certain the jig is giving a true reading – it may be bent – you can flip the wheel over to double check that the rim is centred between the lock-nuts.