It's very easy to find a tritone on a guitar.

Look at this diagram:



The note on the 5th string is C and the note on the 6th string is F#. They are tritones of each other. That shape can be moved anywhere you like up the board.

The lowest you can go is the open 6th string E and Bb on the 5th string. You need only go to A on the 6th and Eb on the 5th to include all possible notes.



If you know your notes on the board you'll easily find the tritone you want. After a while you get to know them anyway.