How do I get a trial of the new iPad Pro??

You can thoroughly explore the iPad Pro and other models of iPad at an Apple Store - where you will have unrestricted access to display models and demo' Apps.

Should you decide to buy directly from Apple in-store or online, in most regions/countries you have a generous 14-days within which you can try the iPad in your environment - and unconditionally return an undamaged device for exchange or full refund.

As such, purchasing from Apple is risk free; if you don't like the product, or would prefer a different model, it can be returned within the 14-day period without question.

AFAIK Apple do not offer free trials. However there is nothing stopping you from asking at your local Apple Store. Nothing ventured, nothing gained.

Good luck.