Making long story about checking the IPv6 PMTU short, here is how you can check the Path MTU for a certain IPv6 destination in the cache:

Linux
ip -6 route get <address>

linux-ipv6-dcache

OS X
netstat -f inet6 -narlW | grep <address>

matjaz@macbook:~$ netstat -narWl -f inet6 | grep 2001:x:e813:
2001:x:e813:a00::d25      fe80::222:56ff:feba:21bf%en1    UGHW3Ii
         0        8   1400      en1   1378

FreeBSD
sysctl -o net.inet.tcp.hostcache.list
(many thanks to Rui Paulo for this hint)

root@FreeBSD:~ # ping6 -m -s 5000 2001:xxxx:e813:a00::d25
PING6(5048=40+8+5000 bytes) 2001:xxxx:e811:b00:20c:29ff:fe78:31fb --> 2001:xxxx:e813:a00::d25
5008 bytes from 2001:xxxx:e813:a00::d25, icmp_seq=0 hlim=63 time=1.088 ms
5008 bytes from 2001:xxxx:e813:a00::d25, icmp_seq=1 hlim=63 time=0.903 ms
5008 bytes from 2001:xxxx:e813:a00::d25, icmp_seq=2 hlim=63 time=0.985 ms
^C
--- 2001:xxxx:e813:a00::d25 ping6 statistics ---
3 packets transmitted, 3 packets received, 0.0% packet loss
round-trip min/avg/max/std-dev = 0.903/0.992/1.088/0.076 ms

root@FreeBSD:~ # sysctl -o net.inet.tcp.hostcache.list
net.inet.tcp.hostcache.list:
IP address        MTU  SSTRESH      RTT   RTTVAR BANDWIDTH     CWND SENDPIPE RECVPIPE HITS  UPD  EXP
2001:xxxx:e813:a00::d25  1400        0      0ms      0ms         0        0        0        0   11    1 3600

Windows
netsh interface ipv6 show destinationcache

win7-ipv6-dcache

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