When you pickup your steel string beauty, make sure it's in tune. Every time. This is the starting point of ear training. Tuning up by ear.
This is where the concept of attention comes into play. There's no trick to this; pay attention to the process when tuning up:
Connect your voice to the instrument and hum the note. Let's give it a shot.
Click for a online guitar tuner
Listen to the correct reference note and try to match the pitch by ear on your guitar. Check your work at the end with a tuner.
Once you get the sound of the low E string down you can use this trick to tune up without a tuner at a party and play "Wonderwall" for all the ladies.