When you change data models on the app engine to add new properties those entries without a certain property are listed with the value <missing> in the online data viewer.
What I'm wondering is how can I write a query to find those entries?
There is no direct way to query for older entities with missing attribute, but you can design data model upfront to support this. Add a version attribute to each model class. Version should have a default value, which is increased every time model class is changed and deployed. This way you will be able to query entities by version number.