I set bit-rate of wireless interface 'wlan0' using
iwconfig wlan0 rate 54M fixed
Now, how do I verify if it was set right?
iwlist wlan0 bitrate
unknown bit-rate information
From the iwconfig man page; emphasis added:
For cards supporting multiple bit rates, set the bit-rate in b/s. The bit-rate is the speed at which bits are transmitted over the medium, the user speed of the link is lower due to medium sharing and various overhead.
Not all NICs/drivers can support setting bitrate via iwconfig, and will operate at their default speed for the negotiated connection regardless of whatever iwconfig has to say about it.
Also, the man page says little about the 'fixed' token, but it looks like you want to use 'auto' instead of 'fixed'.
First things first. 'iwlist'/'iwconfig' and similar tools can display bit-rate of an interface only when it is connected. When the interface is not connected(transmitting), there's no way to determine the rate.
If the interest is in 'how to adjust these bitrates correctly', one may look at: this answer at 'Unix StackExchange'