How do I find my Apple ID connected to my iphone if the phone has been lost?
There doesn't seem to be any easy way to do this.

To do it by trial and error, you can go to the iCloud web-site and login with each of your Apple IDs and try the "Find my iPhone" feature to locate it and/or send a message/tone to it.

If you have any other iOS devices with a similar setup, you can check on them as well.