After intubation, a quick and easy way to estimate ET tube position is by palpating the sternal notch and feeling for the ET cuff.  By holding on to the pilot balloon (balloon used to check if cuff is inflated) and palpating on the sternal notch you should be able to feel the pilot balloon distend if the cuff is at that level (which you should also be able to feel) which is approximately 2-3 cm above the carina and thought to be ideal.

Special thanks to Dr. Irene Osborn and her Difficult Airway Elective for this Pearl

Can also read study published on annals of anesthesia in 1995 regarding this topic.