how can I tell if signal is 2.4GHz or 5GHz

On your Mac.....

Hold down the option key while you click on the fan shaped AirPort icon at the top of the screen. The info displayed will provide details about the connection.

5 GHz might run at 300 Mbps + with a really strong connection, but typically, this number will be in the 200-270 Mbps range if you are within close proximity of the AirPort Extreme with your Mac.

750 MB would be 6,000 Mbps. Since Gigabit Ethernet runs at 1,000 Mpbs, I am not sure what you are referring to here.

The easiest way is to use the AirPort status icon on the OS X menu bar at the top right of the screen. Hold down the Option key before clicking on the icon. Which radio you are connected to is determined by looking at both the PHY Mode & Channel settings. The connection speed is listed as Transmit Rate and it is measured in Mb/s, NOT MB/s.

FYI ... 750 MB/s = 6,000 Mb/s ... which would be impossible with today's Wi-Fi technology. Your 5th generation AirPort Extreme Base Station (AEBSn) is capable of 300+ Mb/s over a short distance. Actual throughput is considerably less.