if you have access to web server, you can send them a link to a picture, or documents, and when they visit that link, you can see the IP address that they are accessing from, and at least show their country, maybe location where they are accessing your links from, if they are not savvy, and with court orders, their internet provider can provide the account information
say if your 'friend' is supposed to be in Europe, and you send them a link to your picture and you see they are opening it in Nigeria, that's a confirmation,
modern email do guard against this somewhat, if you embed a picture in your email, and they're accessing it on gmail, it'll only show that someone from google accessed your picture, but you can send a link to say a document that they must click to download themselves
this is why it's risky to open links or attachment from unknown source