If you are running code versions 8.2 and before, you will need duplicate licenses, one for the Primary and one for the Secondary in an HA pair.
If you are running code versions 8.3 and after, you only need a single license, and the Primary and Secondary firewall in the HA pair will 'share' the features.
This change is outlined in the 8.3 release notes, which you can read here:
Failover licenses no longer need to be identical on each unit. The license used for both units is the combined license from the primary and secondary units.
If you happen to have had two licenses, one for each pre-8.3 Firewall in an HA pair, and then you upgraded to 8.3 or later, then the licenses will be combined. Which is to say, if you had a 5 SSL VPN user license on the Primary, and a 5 SSL VPN on the Secondary, you would now (after upgrading to 8.3+) have 10 SSL VPN users allowed to connect at once. This works for licenses which offer a numbered feature. If your license was something like AnyConnect Essentials, there is no benefit to upgrading (*see note below), since its impossible to enable the AnyConnect Essentials twice.
*There is no benefit to upgrading in so far as duplicating the licenses... I would say, however, there is significant benefit in terms of security and functionality of upgrading past 8.3. In fact, I would suggest to go all the way to 9.x+ to get cool new IPv6 features... but that is completely outside of the scope of your question.
To answer your last question... the output of
show version has changed in 8.3+ and should be sufficient in determine what your firewalls in the HA pair are licensed for or not.