How can I detect successful Javascript execution on a web page?

I'm making an API and part of that API involves appending a bunch of XSS vectors to HTTP parameters.

"><svg/onload=prompt(/OPENBUGBOUNTY/)>

How can I detect whether the relevant XSS vectors which contain JS have been executed or not?

At first, I thought it would be as simple as checking the source for reflection but this isn't a failsafe method and will induce a lot of false positives.

Is there any simple method and efficient method available to deal with this?

The relevant vectors will always be assigned a static text value (ie 'XSS').