How do I know if my router has the latest firmware?

Again in the wrong forum. Please post your questions in the proper forum if the answer doesn't satisfy you. 

https://community.netgear.com/t5/Nighthawk-WiFi-Routers/bd-p/home-wifi-routers-nighthawk

top right corner of your browser gui will have the firmware #. Or your status page for the router. 

Or go to advanced---administration----firmware update. You can auto check for updates. 

You can also read in the manual that I linked to in your other post.