You can check it here: https://www.apple.com/support/iphone6s-unexpectedshutdown/
I tried yours and it comes back with this:
"The serial number you entered is not eligible under this program because either:
- It's not one of the serial numbers in the affected range
OR
- Our records show that your battery has already been replaced. If you paid for a battery replacement service, check with Apple to see if you are eligible for a refund."
I'd recommend setting up a Genius Bar appointment to discuss your options with them. You can still have them replace it but they will charge you the $49 fee now.