How to check where and all specific extension has been used in Thingworx entity?

Hi @Test_user,

An extension can contain various numbers of entities and widgets.

You won't be able to find where Widgets are used, but if you look on any entity, not only extension entities, there is a "View Relationships" Tab, which gives you access to the "Uses this Entity" and "Referenced by" (Help Center link here, a more advanced version of Uses this Entity, which catches many other locations where this specific entity might be used, like Mashups, service code). That's what you should use to find out where that entity is used.