Firstly and most importantly, face facts. You are not a small dog, and while you may pride yourself on your delicacy and finesse in certain situations (ie checking out what’s on the kitchen bench while pretending you’ve really come for a pat) picking pockets is not likely to be one, due to the size of your nose.
First, use your detection skills, and have a good sniff. If there’s nothing worthwhile in the pocket, save yourself the trouble. If, however, you smell treats, don’t hesitate - go for it by shoving your nose as far into the pocket as you can.
Note: while this method has some success if the pocket is attached to a human, it won’t take long for the human to realise you’re there. The best you can hope for is that you manage to score a quick treat before you’re scolded. The method works best if you find a pocket full of treats in a discarded piece of clothing, preferably in a dark room where you will be unobserved. Remember not to noisily smack your lips as you eat; humans are very sensitive to certain sounds and tend to investigate this particular dog noise very quickly.