I agree, it's not a foolproof solution, but the presence/absence of various trackers in their app and website is usually a good predictor.

Furthermore in the name of pragmatism I am not opposed to all ad targeting; some of it is indeed outside of my control. However, there's a difference between sending a name/email address combo once during signup and having the advertising SDK ping the advertiser every single time the app is interacted with (which is what the Facebook SDK does for example); the former only leaks "I use X service" once to the advertiser, the latter leaks a detailed trail of my usage patterns & IP addresses (can be used to correlate where I've been and who I'm hanging out with if their own phones are connected to the same Wi-Fi network and thus share the same IP).