Well, the best way to handle this would be custom license fields: http://wyday.com/limelm/help/license-features/
One way to handle this is to create 2 custom license fields:
1. An "is_trial" Checkbox type custom license field.
2. An "product_expires" Date/Time type custom license field.
So, for the trial version of your app you would set the "is_trial" to checked (or "1"), and for every version you would set the "product_expires" to when that particular version expires.
Does that make sense? So your program would read both those values and you'll know both whether it's a trial, and in every case when the "subscription/trial" ends.