There is a way to do it, and the information is posted somewhere on this board, but I can't find it. What you can do is put each frequency in channels 1 thru 20, using 5 different banks, in your scanner. To clarify this, put the first frequency in channels 1 thru 20 of one bank, then the second one in channels 1 thru 20 of another bank, and so on. Then watch your scanner and when there is activity on a frequency, whatever channel it appears on is the LCN for that frequency. Again, to clarify, if the first frequency becomes active and appears on, say, channel 5 of the bank, then the LCN for that frequency is 5, and so on for the rest of them.
You can also monitor the system conventionally, by simply putting the frequencies in your scanner , in any channels, in the conventional mode, just like you would enter a conventional frequency. You won't, however, be able to determine talk groups and will hear all activity on the system.
All of this information applies to standard LTR systems. There are other types of LTR systems that, I think, can't be monitored, I'm not sure. I'm not really up on the different types, but maybe someone could shed some light on this.