Can someone please provide (or direct me to) me with a clear explanation of

how one can tell which slot an OP is using by looking at the waterfall?
Obviously, there is far more information that can be discerned from the
waterfall than I have understood.

Hi Dave
User Guide 6.3. FT8 describes linkage between the waterfall and Band
Activity window. The frequency Freq value indicates the left hand tone of
the signal. E.g. there is some signal at about 460 to 520 on the waterfall
and in the Band activity there is a message from HB9CQK with frequency 466.
So you need to look both the waterfall and band activity to tell who is who.
Note that you may see some signal on the waterfall, but no corresponding
message and even more often almost nothing on the waterfall, but still a
message.
On the other hand you will try to locate a spot without any signal for your
TX. There may be a free frequency e.g. at 800 Hz.
73, Reino OH3mA