How do I tell if my iPad Pro is 3G or 4G?

There is no 3G only iPad Pro.

All iPad Pros from the very first one in 2015 have supported at least 4G LTE.

https://www.apple.com/ipad/compare/?modelList=ipad-pro-12-9-1st-gen,ipad-pro-9-7,ipad-pro-10-5

Presumably you have a WiFi+Cellular model iPad - as WiFi Only models do not have any Cellular capabilities.

You can find links to the specifications of recent and current iPad models here:

iPad - Compare Models - Apple

Your iPad model can be found in iPad settings:

Settings > General > About >

• Model Name
• Model Number

Assuming that you do have a WiFi+Cellular model - and have added an eSIM or physical Data SIM with an active Cellular plan - whether or not you have 3G or 4G connectivity will be determined by your Cellular Provider and your local coverage.

Your iPad Pro supports LTE/4G Cellular. It does not support 5G Cellular.