You can try:
$ sudo ping -6 -s <size> -M do <destination>
this will check if segment defined in <size> is able to get to the destination
e.g.:
$ sudo ping -6 -s 1452 -M do destination.com
Remember that IPv6 header has 40 bytes by default or more (if options is present) and ICMP header has 8 bytes
This way 1452 in the command means 1452 + 40 + 8 = 1500 bytes