How do I find a wi fi network I told my phone to forget

Scan for it when in proximity, use your camera to scan a QR code for the network if one is available, or ask the folks supporting the network for the info.

If the SSID is hidden, scan with a Mac and some other Wi-Fi tools, as hidden-SSID networks do stick out these days.

Usually easiest to ask the network owners, though.

You don't, until you are in the vicinity of the network again and you join it.