You can do a generic scan for any Gnus backend. Look at
gnus-group-get-new-news-this-group is an interactive compiled Lisp
function in `gnus-group.el'.
(gnus-group-get-new-news-this-group &optional N DONT-SCAN)
Check for newly arrived news in the current group (and the N-1 next groups).
The difference between N and the number of newsgroup checked is returned.
If N is negative, this group and the N-1 previous groups will be checked.
If DONT-SCAN is non-nil, scan non-activated groups as well.
It won't tell you about just the current thread, though. Gnus threading is pretty flexible and happens late in the game, so you don't know what thread the article will go into. For that, you should just do a full refresh like in
M-g (translated from <escape> g) runs the command
gnus-summary-rescan-group (found in gnus-summary-mode-map), which is
an interactive compiled Lisp function in
It is bound to M-g, Z G, <menu-bar> <Gnus> <Exit> <Rescan group>.
(gnus-summary-rescan-group &optional ALL)
Exit the newsgroup, ask for new articles, and select the newsgroup.
...but that complicates things much more, so I would just use