It is possible to change the type of interrupt in the driver to be loaded (if that driver has such support). But if a driver is loaded and if there is a need to know the type of interrupts used by loaded driver, even then it is possible to find that.

Run "vmstat -i". It lists the drivers, their irq number and the number of interrupts taken by them from system start. If the irq number is 256 or more, it confirms that the driver is using MSI/MSI-X interrupts.

 FBSD66# vmstat -i
 interrupt                          total       rate
 irq1: atkbd0                          20          0
 irq12: psm0                            8          0
 irq14: ata0                        63496          0
 irq15: ata1                           47          0
 irq256: nxge0                   32028634        219
 irq23: rl0 ehci0                  125340          0
 cpu0: timer                    292259075       2000
 Total                          324476620       2220