The longer one are the lowers. The lower tusks (cutters)grow continuously and are sharpened against the uppers (whetters)
You can see the black gum line on the tusks. It is common on mounted heads or European skull mounts for them to "pull" the tusks to appear much larger than they were in life, since most of the tooth is below the gum line and inside the jawbones.

When the mouth is closed the lowers should be in front of the uppers. Neither of these hogs as mounted would have been able to close their mouths or eat.
If they have whited out the gum line, you can still tell. If it doesn't look like the mouth will close with cutters against the whetters then they have been pulled.
Here is another good example. You can tell where the lowers are sharped by the uppers, but not matched up in the mount.

To me, this is the equivalent of mounting a big buck, but slitting the skull cap to give it 6 more inches of spread.