Be on the lookout for them every time you tend the plant, when you water when you prune, check the plant thoroughly. It should look healthy at all times.
Leaves and flowers should be checked thoroughly for any signs of damage or uninvited guests.
The quicker you find the problem the faster you will be able to rectify it and the less long term consequences for your tree.
Always give new plants the most careful of scrutiny as they can introduce problems that you have long since eradicated from your existing trees.
If you can isolate it as well for a couple of weeks, this will give any hidden problems, diseases or pests time to manifest more visibly.
When you discover an issue with any of your plants make sure to identify the culprit correctly as this will influence the type of remedial action.