Hi.
This is a common problem and we have discussed it in this thread.
N-formyl is a common impurity in many drug products where the secondary amine reacts with formaldehyde or formic acid present in trace levels in excipients. This will increase on stability. (For eg N-formyl varenicline)
These impurities are monitored in percentage levels in the RS method. Since the magnitude is 100-1000 times higher than the nitrosamine, you have to chromatographically separate it. Also note that the levels of N-formyl impurity at the end of shelf life will be higher so account for that in your separation process.
This impurity can be synthesized or procured. I suggest to get the standard of this impurity and use it in your method development along with the nitrosamine.