Localization V1 and V2 would both exist in the DevTools during the migration. To check whether the file you are touching is migrated or not:

1. Is there a const UIStrings = declare at the top of the file after import statements? ✔️: V2
2. Is there i18n referenced in the file? ✔️: V2
3. Is there ls`` , Common.UISting(), or UI.formatLocalized() in the file? ✔️: V1

Presubmit step would also inform you if there are V1 APIs used in a migrated file.