here’s some more procedure:
1)If you have any optimization tool to plot the measurment graph of rx_level and rx_qual in the same graph, and if you see that rx_qual is poor and rx_level is strong,then you can guess interference in that cell.
2)Get the BER statistics for each TRX of a prticularly cell. Interference affected TRX should have high BER.(But high BER,may also due to faulty HW,so check this first).
3)Perform drivetest, try to setup call both in BCCH carrier and in non-BCCH carrier to see which are affected.
..and…Pix already mentioned to guess quickly probably the presence of interference.