How are people with new IPad Pro getting cellular

Only WiFi+Cellular models of iPad have Cellular capabilities. WiFi Only models do not have any Cellular radio hardware.

If you have a Cellular model, setting-up Cellular service is straightforward:

Set up cellular data service on your Wi-Fi + Cellular model iPad - Apple Support

If you have a new iPad - and are swapping an existing Cellular plan from an old to new iPad, this procedure is equally simple:

Remove or switch the SIM card in your iPad - Apple Support

I want to make sure the cellular works on my iPad Pro 11".

It's about to go off warranty, and I've only used the WiFi so far.

The problem is I don't want to sign up for the more expensive Verizon or AT&T plans.

Verizon wants a start up fee plus the monthly service charge. The start up fee is not worth it for only a month's trial.

I tried Tracfone.com, Simplemobile.com, and Straighttalk.com.

They all say they do not have a plan for iPads.

Is there a plan I could try for a month just to make sure the cellular works on my iPad Pro 11"?

I don't have a use for cellular right now. I would like to use it in the future, but not now.

I just want to ensure that the cellular works.

Anyone have any ideas?

You can add a short-term Cellular plan, to WiFi+Cellular models, directly from iPad settings. Available short-terms plans vary by country/region, but offer the opportunity to activate Cellular when required. These plans are generally limited to a specific volume of data for a specified period of time.

You might also find a Cellular Carrier that might offer a pre-pay SIM option.