How are the pins of a TO device numbered?

 

Usually they aren't, though there are exceptions. Most are labeled according to function on the data sheet.

A voltage regulator, for example, is labeled input, output, and ground.

 

Generally it's 1, 2, 3 from left to right, looking at the pins with the heatsink side down.

http://circuits.datasheetdir.com/97/PS-702-pinout.jpg

 

The datasheet shows a photo and labels which pin does what.

 

The datasheet

Almost every chip has a datasheet. The ability to google "(part number) datasheet" is essential.

 

I get datasheets from www.datasheetarchive.com where I can select from a list of manufacturers. Usually I select the manufacturer who designed the original part because some copy-cat manufacturers make errors or do not provide details on their datasheet.

 

Best to use the data sheet from the manufacturer of the part you have though. Some parts by different makers use a different pin-out. Pins may not be the same connection internally from one company to another on discrete components.