A phone lying face-down is not proof of anything by itself. Here is the honest read on when the phone-flip actually signals cheating, when it doesn't, and the one 60-second check that removes the guessing.

No. Plenty of people keep their phone face-down out of habit — bright screen at dinner, focus mode enabled, distraction avoidance. The signal is the change, not the position. If a partner used to leave the phone screen-up and now consistently keeps it face-down, the change is worth noticing.
When it clusters with three specific patterns: the flip is triggered by specific notifications (particular tones or vibration patterns), the phone starts leaving the room with them (bathroom, walks outside, charging in a different room), and passcode habits have changed (angled away, Face ID disabled, a new passcode).
The pattern maps onto four categories: an active affair, micro-cheating, non-romantic secrets (financial stress, gambling, addiction recovery, family drama), or a boring habit shift (new job, new phone, Focus mode). The bottom two are why the signal alone is not proof.
A photo-based partner check answers the specific question 'does this person have a hidden dating profile' from outside their phone, in under 60 seconds, without any notification reaching them. The result either ends the suspicion or gives a concrete next step, without needing an accusation-based conversation first.
No. Installing monitoring software on someone else's device without their consent is illegal in most US states under state wiretap and computer-fraud statutes, and any evidence collected that way is typically inadmissible in divorce court. It also almost always gets discovered, which ends any conversation about the underlying issue.
Watching indefinitely turns a two-week check into a six-month unresolved doubt and distorts the read on ordinary behavior. Once the pattern has persisted for a few weeks and clusters with two or three co-signals, the useful move is to switch from observation to a concrete check.