How to look for a text/string on the page
If you are relying on contents of the page, either text, a data-attribute, or a variable to manually activate an experiment, you will want to learn jQuery selectors so you can target the specificelement(s) on the page that indicate the targeting condition.
Analytics and Testing Guru
Could you provide a little more info on the scope of this? You're process of finding this text/string is going to be different if it is dynamically changed after page load (either by a timer or user input)
@JasonDahlin's reference above will be useful if the text/string already exists on the page onload, but if it doesn't you're going to have to take a different approach.