a) X windows: xditview and xtroff (X11R5 contrib directory, xtroff
       also from ftp.cs.toronto.edu) and gxditview (from groff) will
       preview ditroff format (groff requires gxditview), including
       psroff 3.0's ditroff output from CAT troff.
    b) Display PostScript (ie: AIX 3 & DEC dpsexec, NeXT,
       Sun Pageview, Ultrascript, Ghostview plus GhostScript): any
       PostScript generating troff.  (Theoretically at least.  May need
       a teensy bit of PostScript hacking to get a reasonable pager
       (psroff3.0 has this built in).  Pageview and Ultrascript have some
       limitations so may not work with a specific PostScript
       generator).  Pageview gets very confused about page boundaries
       unless the PostScript is DSC compliant and has "%%Page:" comments
       in it.
    c) Others: You can probably find a PBM converter for it,
       so use a Laserjet-generating troff configuration plus hp2pbm.
       (eg: Sun rasters, 3b1 screens, EGA, VGA, GIFF, TIFF etc.)
       I preview on X by generating PCL with psroff, converting
       to PBM with hp2pbm, then display with pbm2Xd (part of hp2pbm)
       and xwud (part of X windows).
    d) Others: You may be able to find a DVI driver for it,
       so use a DVI-generating troff configuration (namely,
       groff's ditdvi) and use the appropriate dvi2<whatever> filter.
    e) AIX 3.2 has a good AIXwindows 1.2 and Motif1.1 based
       previewer called "xpreview".