How to identify gear tooth wear and tear

Change the oil in your gearbox, check for alignments and grease bearings according to manufacturer. Short of this the only way is to watch for metal filings in the oil, listen for abnormal sound or vibrations. When there is no load on the output shaft and the drive belt or chain is removed, you can " push/pull lift/lower turn right/turn left on the shaft and feel for any free play or movement of the shaft. You can also watch for any oil seepage through the shaft seals.

To identify how much wear has occurred you would need to measure the wear on the gear and compare it with a new or your original ( if you have them ) specs.

There are all kinds of ways to monitor the conditions present, such as thermal imaging when it's new to get a baseline reading and taking measurements at specific intervals, but your ears eyes and touch can tell you quite a lot without having to revert to exact measures. Unless exactness in your running condition is absolutely necessary. These may be of some use!