You can look at the SPI label located inside the glovebox door to see if it is listed there. This is the SPI label from my donor '90 Suburban:
GT4 = 3.73 Ratio
GT5 = 4.10 Ratio
GU4 = 3.08 Ratio
GU6 = 3.42 Ratio
Short of that, you could open up the axles and take a look at the numbers on the ring gear. 41-11 is 3.73, 41-10 is 4.10, etc.