There is not a way.
Unless that same message you sent gets forwarded from recipient 1 --> recipient 2 --> back to you (you would check headers), there is not a way. In order for this to even be an option, whoever your recipient uses as an email server/client, would have to be built with this functionality.
- Below is merely added to show a very limited potential to track emails being forwarded.
You could potentially reference an embedded image on the internet (something you can track), AND* if the "forwarded recipient" allows images to be viewed, you MAY* be able to see the IP's that have accessed your image you referenced in the email.
AND* = Image viewing is rarely ever enabled. User has to allow this to happen on a per-email basis.
MAY* = Now you run into the hassle of trying to understand your recipients network and if many people share public IP's (think home networks or small businesses, cheaper for internet provide to assign one IP to house/building)
You would THEN have to go into the hassle of tracking down an IP to a person.