Checking RAIM is fairly straight forward. For a primer on RAIM, see: Procedures & Techniques / Global Navigation Satellite System (GNSS) & Global Positioning Satellite (GPS) System.
What follows comes from the sources listed below as well as my comments and techniques shown in blue.
[G450 Aircraft Operating Manual, §2B-17-30]
Photo: MCDU NAV Index 1 page, from Eddie's aircraft.
Select the NAV Index page by pressing the NAV key.
Select POS SENSORS (LSK 4L)
Photo: MCDU Position Sensors page, from Eddie's aircraft.
Select GPS 1 STATUS (LSK 5R)
You could check the status of the other GPS as well.
Photo: MCDU GPS Status page, from Eddie's aircraft.
Select PRED RAIM (LSK 6R)
Photo: MCDU RAIM page, from Eddie's aircraft.
You now have the RAIM check for here and now, what about there and then?
Select PILOT SEL (LSK 1R)
Photo: MCDU Pilot Select RAIM page, from Eddie's aircraft.
Enter the PLACE at LSK 1L and the time at 1R and you will be greeted with the RAIM prediction based on the downloaded GPS almanac.
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Gulfstream Flight Management System Quick Task List (FMS QTL), Rev 1, Aug 20, 2012