The easy part is finding the SNMP variable -- it's here:
EtherLike-MIB::dot3StatsDuplexStatus.1 = INTEGER: fullDuplex(3)
And it would be great if one could create an interface group that tested for that, but the group criteria don't currently include it as a choice.
That being said, there already is a canned issue that displays every 100/half port -- it's "Incorrect Duplex Setting". It's been broken since it was created because the only thing it looks for is 100/half -- it doesn't look for multiple downstream MACs that occur with a hub. But until it is improved, it does exactly what you want. :)
- Marty