There will still be a bandplan. Every RF channel (freq pair) assigned in a Moto system has to be assigned a bandplan entry, even if it's the only RF channel referencing that entry.
You gotta sniff it with a system decoder such as Pro96Com or Unitrunker to get the bandplan... then you should be able to figure it out. The split is listed in the bandplan based on difference from the base radio TX freq in MHz. If you know what the output frequency is and the bandplan line referenced for that RF channel, you can get the input by applying the split in that line.
So, just for wild weird example, if the system reports output freq 154.010 as using bandplan line/entry 2, and the bandplan reads (abbreviated):
1 | whatever base freq | -1.5MHz offset
2 | whatever base freq | -1.1MHz offset
by applying entry 2 to 154.010, you get an input frequency of 152.910 MHz.