In a dired buffer, how can I determine whether the file under the point, i.e. (dired-get-file-for-visit), is local or remote? Is there something like (dired-local-file-p) or maybe (tramp-local-file-p file)?
I would like to call different functions for visiting local and remote files.
If you simply want to know whether the file is accessed through a remote acces method, you can call file-remote-p (built into Emacs) or Tramp's similar function tramp-handle-file-remote-p. Both functions return nil for an actual file name and the method prefix (as a string) for a file accessed through a remote method.
This may or may not be what you're after. Files on a remote filesystem mounted by the operating system (NFS, Samba, Windows shares, SSHFS, etc.) will be reported as nil; you can call df to get more information if you have unix tools available. Contrariwise, local files accessed through privilege escalation (e.g. /sudo:: or /su::) are reported as non-nil. You can call tramp-dissect-file-name and the tramp-file-name-… function family (all defined in tramp.el) if you want to analyze the method further.
I realize you're asking to detect tramp buffers vs, say, files on a remote mounted filesystem, but for posterity this might be useful to other searchers:
(defun buffer-is-on-local-fs-p ()
(and (buffer-file-name)
(let ((local-path (chomp (shell-command-to-string
(concat "df -l \"" (buffer-file-name) "\" | awk '/^Filesystem/ {next}; {print $6}'")))))
(not (string= "" local-path)))))
This uses unix's df to identify the filesystem hosting the full path of the current buffer, and then makes a best-effort to determine what its mount type is. It's not perfect, but it's served me pretty well (I used to use it to gate VC mode funcionality, which was painfully slow on remote filesystems).
buffer-file-name to extract a filename, but you could just as easily pass in a path and use that.