Flight Crew Air Transport Pilot (ATP)
Confusion
Deviation / Discrepancy - Procedural Clearance; Ground Incursion Runway
General None Reported / Taken
Human Factors
We taxied out from the FBO via our issued ATC clearance.  The clearance was taxi to  Runway 33 via A Taxiway and cross Runway 25.  As we taxied across Runway 25 I saw a sign that said  approach for Runway 33 and I believed that I saw hold short lines.  At this point I told my PIC that he was approaching the hold short lines for Runway 33; the PIC replied that those where not hold short lines.  A few seconds later the PIC stopped the airplane because he started having doubts as to if we had crossed the hold short lines.  As we discussed the issue the Tower cleared us for take off.  We never saw any other hold short lines as we taxied onto the runway.  If you review the airport diagram  its difficult to see if there are any hold short lines from the point that we taxied onto the runway.   At this point we are not sure if we crossed the hold short line.  Next time I will be more aggressive in regards to stopping the airplane.
G200 First Officer questions whether the hold short line for Runway 33 has been crossed as the crew taxi's to that runway via Taxiway A at DET.