{linking} when the result is {link}

I was wondering if there was a way to include a mark or note when {linking} was returning the value of {link}. {link} in my case will always return an external link, to another site. I wanted to add an icon or a note (ie “external link”) when using {linking} returned {link} instead of {details}, so that users would know if they are being directed to another site when clicking.
I realized that I cannot add a php condition to the template.
Do you have any suggestion?
Thank you!