Will payers know how much time I spent?

When you submit a CPT code for E/M office visit services, the payer doesn't know if you chose the chose by MDM or Time. It's if/when you get audited that you will need to show documentation that supports the code you chose.

Best practice is to document "which way" you chose your CPT for reference in the future because it's likely you won't remember how each visit was coded. Your note documentation should make it fairly obvious.