I know I'm reviving an old thread but in case anyone else searches and finds this like me, the only sure fire way I've found is to either do a "show slots"....
#show slots
Slots
------
Slot Status Card Type
---- ------ ---------
1 Present A3400-32
...or if that doesn't please you do a show license and a show ap license-usage together to see if there are more AP licences available than installed.
#show license verbose
License Table
-------------
Key Installed Expires Flags Service Type
--- --------- ------- ----- ------------
xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx-xxx 2015-01-01 Never E RF Protect: 32
12:39:24
xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx-xxx 2015-01-01 Never E Access Points: 32
12:39:42
xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx-xxx 2015-01-01 Never E Next Generation Policy Enforcement Firewall Module: 32
12:39:58
License Entries: 3
Flags: A - auto-generated; E - enabled; R - reboot required to activate
#show ap license-usage
Total AP Licenses : 64
Remote-Node AP Licenses Used : 0
AP Licenses Used : 19
as noted above, a show inventory doesn't actually do it, just returns the base model.