How to check if a trademark is already in use?

Do a quick search via USPTO

1.  Go to https://www.uspto.gov/ and click on "Trademarks" 

2.  Under "Application process" select "Searching trademarks"

3.  Click on "Search our trademark database (TESS)"

4.  Open link in new tab for all items under "Select a Search Option"

5.  Put in your trademark in the Search Term box and click "Submit query"

Goal is to have an original trademark so you want to have 0 (zero) search results in any of your searches

You want the results to look like this:

NOT like this: You want to have an original trademark

It is still advised that you get a professional just to be sure but this is the most basic way of checking if a trademark is already taken.