I would advise checking
Realtime Trains late on the evening (ie after 2130) before to see which trains have been activated (have a time in the "Act Dep" column) and therefore have a chance - and only a chance- of running of running.
Any paths between the following will likely be 73s:
Hither Green Down Recp to Hither Green Down Recp - normally pair TnT test coaches. These will normally be WTT "Q" (Runs as Required) paths.
Any moves between Tonbridge West Yard and Eastleigh are also likely to be 73s. Sometimes WTT, sometime STP
If you only want to see some in the sidings, get a train from London Bridge to Tonbridge (Via Sevenoaks). You should see at least one on your left between Hither Green and Grove Park. Also there should probably be one or more stabled in Tonbridge West Yard (on your right as you approach Tonbridge from Sevenoaks). If you want to, you can leave the station and
walk a short distance to a footbridge over the yard to allow you to get a better look. The path to the bridge is slightly up and to the left of where the B is on the map
Or, if you want definite answers you can always check what
Mr Freightmaster has to offer
Here's a few of my own photos of 73s around Sevenoaks and Tonbridge
Joe