Purchased a new moto g4 plus to replace a moto x 1st gen. Phone arrived with sim not installed. Does this mean I am on the GSM network. How can I tell which network I am on from my phone? I know I have lousy service available with both networks.
The fact that the SIM wasn’t preinstalled suggests your new phone is provisioned for GSM rather than CDMA coverage.
You can confirm by installing SignalCheck Kit as referenced by @mb2x. SignalCheck Lite will show whose cell towers you are connected to.
Alternatively, you may dial ##786## (don’t hit send) as if you were making a phone call. If nothing happens it’s GSM. If you’re presented with a “SprintDM” menu, well it’s explanatory.