You can get the typical settings. For a general user there should not be the need to know the load for a NIC. You would need a 'extender' that would be constructed with the means to check the current via a sense. Not every one would want to purchase a 'pci/pcie' extender to just measure the load. The specs do provide enough detail to setup a good idea as to what is used typically.
We're not threading a needle here!
If the OP needs the load then he/she should know how to measure and DAS the data for computation over a period. Then conclude a load for the NIC over the measured test.