Let’s assume that:

you want to chat with me;

you only have my publicly available email address (`sam@rfc1149.net`);

you suspect that I also have a gmail account linked to the `sam@rfc1149.net` address;

you want to find it so that you can initiate a chat.

It is easy. Create a new Google Site, and enter the Site settings/Sharing tab. Add my name as a viewer and click Invite these people.

Then select Skip sending invitation so that I am not notified.