I have played for 16 years now and YES, poker tells can mean the difference in making the money and not , winning or losing a pot, or winning or losing a tournament.
I WATCH EVERYTHING! period.
Common and best way to pick up tells.
Before betting, look to the left. Grab chips and look left. If someone grabs chips and they often do before you even get your chips into the pot, then they are strong and were very happy to not only call your raise or any raise , and will likely be in the top 5 hands.
Grab chips and look around the table. People pick up their cards, pick them up and shuffle them, turn them to the side, likely will be folding to any raise and are weak.
Do not look at your hand until the action is on you. Often stronger hands will be aggravated that you are taking a few more seconds to call , raise or fold. Look for it. Invite it.
Anyone looking at their hands when multiple suited cards hit will look back at their hole cards. If they do , they are on a draw and arent mentally aware enough to know what two suited cards they have. We jokingly call it a Heart check, spade check ect .
Possible straight out there? Watch and see who looks. They are on a draw.
Anyone who says tells cannot make you money, or arent important I would seriously discard that information.
What you DO with those tells , matters as much as seeing them and identifying them and using them to your advantage.
Best thing I can tell you is, stay out of table talk. Dont talk strategy and pick the strongest player at the table and focus on their tells. Do they slam chips when they are bluffing? Do they neatly stack their chips or do they keep a loose pile .
Tells can be the difference in laying down AK against a dry board against someone who flopped a set.
Hope this helps.