I'm not aware of any out of the box ways to easily enable this. I think you could however use a macro condition to check the current page / coockie / setting / etc.
You can configure the macro here:
- Select the Free shipping offers Module
- New Free shipping offer
Free shipping conditions
- further conditions block
- click Edit
- and move to the code Tab
You could create an A/B test that has different buttons to navigate to your checkout for instance. And let that buttons navigate to different page, or set a cookie, or set a flag on the current customer object/ or etc..
And in the macro for the free shipping offer let this difference be the condition.
Hope this helps you!