Generally if the RSRP or RSSI increases significantly that is a dead give away you are on a stingray. You don't really need an app to see that (google how to see it for your phone type). Of course, it is always possible that your carrier just turned up a new site closer to you but that is not something that happens often and you can usually notice it.

Some comments about mm-wave 5G, keep in mind that mm-wave != 5G. But on mm-wave stingray becomes more difficult due to the high directionality of short wavelengths. But at lower bands, there is no advantage of 5G over 4G in terms of resilience over stingray types of attacks (which are basically a L1 middleman attack, similar to age old wifi types of spoofs).