For DHCP-based Device Type Detection, the NAC or Access Control Engine appliance will need to see the DHCP handshakes from the DHCP client and Servers. This is typically done by forwarding these frames to the appliance via the IP-helper in capable routers. The NAC will not reply back, it is only in a passive listening mode.
In the DHCP Packet, option 55 needs to be present in order for the NAC to determine the device type.
Option 55 will have a series of numbers in it that are generally accepted as a unique device type. Option 55 decodes as "Parameter Request".
Note that if NAC rules are built with Device type rules in them, they are occasionally subject to a change in the option 'fingerprint' that sometimes occurs during patches and upgrades. If the appliance is out of date, or the device software is newer, then this OS detection may not work.