Methods to detect calls being routed through Stingray devices?

Can't see anything for iOS but there is an app for Android that claims to be able to do this. However, it only works with a specific chipset and only on a rooted phone. So it is very unlikely that you could do this on an Apple phone. Another app is in early development.

There are also dedicated systems for trying to track stingray use. A "homemade" example is here.

In theory, you might be able to track use at a static location by knowing the ID's of the local stations so that you can spot when a new station ID appears.