Ah, POS=Point of Sale. That's not what I thought you meant when I read your first post.
There may be another way to do this but the way I'm familiar with is installed by the POS software. It drops some code into the MBR (Master Boot Record) that looks for a sequence of keys and allows either the desktop or invokes the POS software at boot.
Assuming that it does not display a graphic right after POST telling you to press certain keys, then I think the only way to know would be to install that POS software on another computer and the install routine may tell you which keys, or the manual that came with it would, or the software vendors web site may have tech support.
Last time I ran into this the POS software had to have the UPC wand connected or all you'd get was a black screen...wouldn't go any further then a bit past the post.
Maybe someone else has a better take on this. What's the brand or who is the vendor of the POS software?