Open your airport utility on a Mac.
Hold down the Option Key and double click the icon.. this will bring up the full summary page. The top line will identify the model.

On the base of the router there is a number.. eg A1408 Extreme which is model I am using here.. Gen 5.
It is written on the paper label.
This is from the Time Capsule..

And even older ones it is molded into the rubber.

If you have tall one.. just say tall.. we know the model.. It is written on the base in 4pt type black on black.. so without going blind and in my jewellers head set I can read it.. if the light is good and you hold it to the East with strong west wind... tilted at 35deg to the earth axis
The older ones are flat.. newer one is vertical.. very obvious to see the difference.