Short answer: no app can quietly hand you an adult's text messages, and in most jurisdictions trying is a crime before it is a technical problem. If you want to know what is actually happening, the routes that work are a conversation, a transparency arrangement you both agree to, or a licensed investigator — not a covert install.
Why the technical route fails even when someone sells it to you. Modern Android and iOS both seal the message store behind the device passcode and per-app encryption. Any service promising "read her texts remotely, just enter her number" is showing you sample data, or it is pushing a stalkerware payload that needs physical access and Play Protect disabled. On an adult's phone that is interception under 18 U.S.C. 2511 and the state privacy statutes built on it, it is a stalking charge in many places, and it is routinely excluded from divorce evidence.
What you can legitimately check. If the line sits on your carrier account, the usage portal and the monthly bill show every number texted and when — metadata, not content — and that is usually enough to answer "is there a new number in her life." Every major US carrier exposes it under Usage or Call Detail, free. If you both want transparency, a mutually installed location-sharing app (Google Maps location sharing, or a family locator) gives you something with consent built in, which is also the only version a court will look at.
Where our own dashboard fits, honestly. Ultimate Phone Spy runs on Android 4.0+, Windows and Mac, installed with the user's knowledge on a device you administer — a child's phone, a shared family PC. On those it shows SMS threads, call logs, location, web history and installed apps in one panel. Plans: https://ultimatephonespy.com/pricing — requirements: https://ultimatephonespy.com/system-requirements. It does not cover iPhone, there is no remote install, and it is not something you can point at an adult's handset without them knowing.
If the concern is a child in the house rather than your wife, the free first-party tools cover most of it: Google Family Link on Android, Screen Time on iOS, your carrier's family locator.
If threats or coercion are behind the question, a lawyer or licensed investigator gets you further than any software.