Use _pdriver to determine the current print driver:

? _pdriver
// With an Epson FX 80, the response is:
// EPSON9,Epson FX-80
// With an HP Laserjet running postscript, the
// response is:
// pscript,HP LaserJet IIISi PostScript

You can set the print driver with _pdriver:

_pdriver="pscript"