How to tell if my iPad is connected to WiFi using WPA3, WPA2, or no security at all?

If a Wi-Fi network has a lock next to its name, it's a password-protected network.

https://support.apple.com/en-gb/HT204070

This is the only information given by iOS by the Settings app.

iOS APIs don’t provide this information to third party apps. NEHotspotNetwork provides various network information as described in that Apple documentation page (accessible if the app has the entitlement) but not the network security.

If your iOS device is jailbroken, you can install GoodWifi. This shows the network security in the list of networks in the Settings app.

Actually it’s possible to check this without jail breaking. See: https://download.developer.apple.com/iOS/iOS_Logs/WiFi_Logging_Instructions.pdf

Download and install the Wi-Fi for iOS profile without needing to reboot the device and you will be able to view diagnostics page in the Settings. https://developer.apple.com/bug-reporting/profiles-and-logs/?platform=ios&name=WiFi