isrunning-p.el

(defun darrik/isrunning-p (argv)
  "Return `t' if `argv' is running."
  (or (listp argv)
      (setq argv (list argv)))
  (dolist (pid (list-system-processes))
    (let* ((pdata (process-attributes pid))
           (x (assoc 'comm pdata))
           (exe (cdr x)))
      (when (member exe argv)
        (return t)))))

(darrik/isrunning-p "emacs") ; t
(darrik/isrunning-p '("emacs")) ; t
(darrik/isrunning-p '("emac")) ; nil
(darrik/isrunning-p '("emac" "zsh")) ; t

Don't forget to add ".exe" if you're on Windows.
(darrik/isrunning-p "emacs.exe") ; t