your answer is hidden in an example inside a text about target-pane :
target-pane (or src-pane or dst-pane) may be a pane ID or takes a similar
form to target-window but with the optional addition of a period followed
by a pane index or pane ID, for example: ‘mysession:mywindow.1’. If the
pane index is omitted, the currently active pane in the specified window
is used. The following special tokens are available for the pane index:
You can test it by creating a background/daemonized tmux session named $SESSION like so :
tmux new-session -d -s $SESSION
you can monitor this session in another terminal by using :
tmux attach-session -t $SESSION
and then using this command from the "non tmux" terminal
tmux send-keys -t $SESSION:0.42
and then if the pane 42 doesnt exist, tmux will complain.
Now by using a test string, like so :
tmux send-keys -t $SESSION:0.42 "test" C-m
You can pinpoint the exact pane interactively if you don't know the exact pane number that you want.
On a side note, you can display pane numbers inside a tmux session by entering (listed in the key bindings session of the documentation you linked)
C-b q Briefly display pane indexes.