Turn on your tuner and set it to play a consistent, steady tone.
Play the same tone that is produced by the tuner on the trombone.
Listen to how fast or slow the interference beats pulse between the tuner and the trombone pitch. The faster the beats pulse, the further the instrument is from being in tune. As you get closer to the correct pitch, the beats will slow down.
Determine if you are sharp or flat by intentionally bringing the pitch of the instrument down by relaxing the tension in your lips. Listen to what it sounds like to be flat. Do the opposite to identify sharp pitch. Practicing this technique daily will open up your ears and make it easier to identify when you are out of tune.