Okay, I think I've got the final answer. This seems to hold true in the cases I've examined.

- Get the properties of the device in question.

- Go to the "Details" tab.

- Pull down the list of properties and find the one labeled as follows:
{3AB22E31-8264-4b4e-9AF5-A8D2D8E33E62}[1]
(There may be several of these properties, you want the one with [1] at the end).

- The value of this property (which may show up in hexadecimal) is defined as follows:
0 DevProp_PciDevice_DeviceType_PciConventional
1 DevProp_PciDevice_DeviceType_PciX
2 DevProp_PciDevice_DeviceType_PciExpressEndpoint
3 DevProp_PciDevice_DeviceType_PciExpressLegacyEndpoint
4 DevProp_PciDevice_DeviceType_PciExpressRootComplexIntegratedEndpoint
5 DevProp_PciDevice_DeviceType_PciExpressTreatedAsPci
6 DevProp_PciDevice_BridgeType_PciConventional
7 DevProp_PciDevice_BridgeType_PciX
8 DevProp_PciDevice_BridgeType_PciExpressRootPort
9 DevProp_PciDevice_BridgeType_PciExpressUpstreamSwitchPort
10 DevProp_PciDevice_BridgeType_PciExpressDownstreamSwitchPort
11 DevProp_PciDevice_BridgeType_PciExpressToPciXBridge
12 DevProp_PciDevice_BridgeType_PciXToExpressBridge
13 DevProp_PciDevice_BridgeType_PciExpressTreatedAsPci