How to measure BW of IF filter in a LowIF transceiver

In a Low-IF transceiver, the RX side IF bandpass filter is usually intergrated into the IC. Does anybody know how to measure the bandwidth of this IF bandpass filter?

Having a full functional receiver (but no internal access to the band-pass polyphase filter) you can measure the shape of the filter plotting Frequency vs Input Level for an arbitrary BER.
For example: for each frequency step increase or decrease the input level to reach 1% BER, and record the level for that frequency.

Thanks. It is a good idea.

Another way to do it is if you can set the chip in a test mode and have access to the IF filter output:
Sweep the (RF) input frequency and plot the frequency (IF) response.

A.T.B

Maybe due to the IF filter is integrated and don't need external component. I found the most of Low-IF transceiver don't have external pin for the IF filter output. Maybe some secret test mode can access the output, but generally not mentioned in data sheet. It seems that only indirect method can do it.