The best solution might be to use a spectrum analyzer and look for the other spike that pops up at the
same time as the interfering one does. The spectrum analyzer gives you the ability to monitor a wide
band of frequencies at one time.
Unfortunately there is no guarantee that there is any predictable relationship between the interfering
transmitter and your repeater frequency. But sometimes you can work it out: often transmitters
multiply by some convenient number like 6, 8, 12, 15, 16, 18 or 24 to reach their final frequency.
For example, a transmitter on 158.56 MHz that uses a multiple of 12 would have a fundamental at
158 / 12 = 13.213 MHz. The 11th harmonic of this frequency comes out on 11 * 13.213 = 145.34.
Since most paging transmitters near the 2m band are in the 150 MHz range, try multiplying your
repeater frequency by 6/5, 8/7, 12/11, 15/14, 16/15, or 24/23 and see if there is anything close
to those frequencies. You might also be hearing a sub-harmonic from a pager in the 450 band.
Otherwise, track down the signal and look up the site owner of wherever you end up and explain
the situation.