One of the best times to do the bubble test is like pulling Lee hill(it sounds like you may live in that area). When you see the high temps. Pull over(in a safe area), rev the engine to 3-3500 rpms and hold the revs for a few minutes(usually best to have a helper to make it easier). See what you can see.
Anyhow, the other way is to make sure you truck good on coolant, run until the T-stat cycles a few times. Then hold the RPM at 3-3500 for 4-5 min or less if bubbles appear.
It sounds like you burped the system well.
Do you have a stock T-stat in the truck?