On a twisted pair Ethernet connection, the only difference between a full duplex connection and a half duplex connection is the rules the devices on each end use when transmitting.
With both full and half duplex two pairs of wire are used, one for transmit and one for receive. One method for determining the duplex setting is to look at the Fast Link Pulse sent by each end. This will only be present if the nodes are set to auto-detect. Additionally, the analysis tool must be able to read this Fast Link Pulse. The only tool I know of that will read the FLP is the Fluke NetTool.
If the nodes are forced to full or half duplex, there is really no way to determine the duplex settings by looking at the packets on the wire.
mpennac