Actually if I recall it's:
gear ratio = speed / (wheel circumference x cadence)
With gear ratio = chainring cogs / cassette cogs
So, to determine which sproket you're on, you can do the maths as follows

, or better yet, look down on the cassette!
T = R x W x C x 60 / (S x 1000)
With
T: cassette sproket number of teeth
R: chainring number of teeth e.g 34 teeth
W: wheel circumference in Meter. E.g 2.32m for a 29" mtb wheel
C: pedalling cadence in RPM e.g 100 RPM
S: bike speed in km/h, e.g 20km/h
With the above example, the calculation would give:
T = 34x2.32x100x60 / 20×1000
T = 23.8 teeth
On a SRAM GX 10-52T cassette with the following sprokets: 10, 12, 14, 16, 18, 21, 24, 28, 32, 36, 42, 52
And with Gear 1 being the largest 52 teeth sproket,
you would most likely be riding in Gear 6 (24 teeth)