I have machine with multiple NICs that can be connected to one net, to different nets or every other possible way. Python script that uses requests module to send POST/GET requests to the server is ran on that machine.
So, the question is next: how can I know in python script from which interface requests will be sent?
Here there are two different answers. The one is for the case where you want to specify the NIC to send the request and the other is for what you're asking: find the correct NIC. 
For the second answer, I can only say that it depends. Are these NICs on the same network / subnet? Are they bonded? 
If they are on a different network then you can know the host IP Address and use the computer's routing table to see which NIC the packet will go through. If they are bonded on the same network then the request will leave from the bond interface since it is (normally) the one with an IP Address. If they are on the same network and have a different IP on the same subnet then it depends. Can you provide some more information in that case?
Python 3: how to get nic from which will be sent packets to certain ip?