How to know if public WiFi is secure?
Is there an easy application or web-app to determine if a public wifi hotspot is predatory or legitimate?
What is your threat model? If you’re worried about some rando capturing the airwaves and snooping on your traffic, then you’re secure as long as you’re connected to a WPA2 or WPA3 encrypted network, which pretty much every hotspot supports. If you’re worried about whether you can trust the hotspot itself, then no - trust is a social problem, not a tech problem.
Best assume it's not and use a VPN.
You know it's secure if your trusted and secure VPN can connect through it.
Otherwise, there's really no way to know what they are doing behind the scenes.
What's your threat model? The vast majority of websites and services use HTTPS or some other protocol over TLS so the data will be encrypted & authenticated regardless of the security of the network itself.