The check for a leaking o-ring is easy. Wet your finger and hold it tightly over the air intake hole. Then put your mouth on the drip tip and create a vacuum with a hard hit. Then, while holding the vacuum put the tip of your tongue on the hole in the drip tip to seal it off. The vacuum should hold for at least 10 seconds. Theoretically, the vacuum should hold for much longer if the o-ring seal is good. When you pull your tongue off you should sense the air rushing back into the tank.
With some tanks you might cause juice to feed into the chimney and up the tube to the drip tip if you pull too much vacuum. Start with a medium vacuum and go from there. If it won't hold full vacuum for at least 10 seconds, then replace the o-rings. The chimney o-ring under the drip tip is the most likely suspect, but the one on the top cap could also be the problem. The chimney o-ring isn't the easiest to replace, but it goes in easy if you slide one side in, then use a miniature Phillips screwdriver to walk the edges around the inside to seat it while guiding the outer edge in with a thumb and preventing the other edge from walking out. Once you get good at it you might be able to just use your thumb to pop it in.
O-rings can buckle if they are dry when you screw on a part. I always put a drop of juice on the o-ring and smear it around before assembling the parts.