As Brian says, measure the OD of the gear on the Leadscrew.
Measure the OD of one of the other gears, say a 60T ( In both Imperail and Metric units ) Add 2 to the tooth count, so as an example, 62.
If you divide the 62, by the OD in inches, if it is an Imperial gear, you should get a whole number like 14, 16, 18,or 20, possibly. That number is the Diametric Pitch
( i. e. 62 / 3.1 = 20. So a 127T, 20 DP gear would have an OD of 6.45 " )
If it is a Module gear, the OD in mm, divided by Tooth Count + 2 ) should give a number like 1.25 or 1.5.
That is the Module Number..
A 1.5 Module, 62T gear would have an OD of 96 mm. A 1.25 Module, 62T gear would have an OD of 80 mm
Armed with the DP or Module, measuring the OD of the gear on the Leadscrew and applying a similar calculation should tell you how many teeth are on that gear.
HTH
Howard