How to know, how many GPUs are working and what's their frequency?

AMD Overdrive shows 1 GPU
Device manager shows 2 GPUs
GPU-Z claims I have 3 GPUs in CrossFire mode
Catalyst Control Center shows 4 GPUs in tray menu
OverDrive5 lists 5 GPUs.

AMD OverDrive says the GPU has frequency 1300 MHz
GPU-Z says I have 1200 MHz
OverDrive 5 says I have 2 GPUs at 0 MHz and 3 GPUs at 1300 MHz.

How to know whether all GPUs are on and what frequency they are?

Try going to Device Manager and click on the Display Devices tab then it will show how many GPUs are installed on your computer.

But Device Manager has no tabs.