I would like to start looking at my taxes for next year. I have some different things I will be dealing with. Can I do a practice return for 2019?

[VolvoGirl]

If you are using the Online version, do NOT change anything on your return.  You would have to start with a new account and do a test return.  You don't have to pay unless you want to print it out.  

Or Here's the link for the current TaxCaster tool:
https://turbotax.intuit.com/tax-tools/calculators/taxcaster/
 
How to start another return in the Online version
https://ttlc.intuit.com/questions/1894512-how-do-i-start-a-second-return-in-turbotax-online

There is one thing you can do in your  current return.  You can go though the Estimated Tax section which will prepare estimated payments for next year.  That may help you calculate what you might owe.  You can just type W4 in the search box at the top of your return , click on Find. Then Click on Jump To and it will take you to the estimated tax payments section. Say no to changing your W-4 and the next screen will start the estimated taxes section.