I work in a charcuterie kitchen and cube about 200 lbs. of pork each week for sausage. Here's my two cents on finding them in boneless butts...we use butts that have the paddle bone removed but still have the flap attached, so YMMV depending on the cut you buy:
1) if you are standing at your cutting table, turn the butt so that the money meat muscle is facing left (9 o'clock position.)
2) if you then cut the butt into 1.5 inch steaks (i.e. you cut the butts from 12 to 6 o'clock), you will find MOST of the glands lurking in the fat at the tip of the steaks--the tip furthest from you. Usually, the 1.5 inch cut will expose enough of the fat to allow you to simply lay each steak on its side and see a gray mass or see where you've sliced right through a gland.
3) Simply trim away and you're all set.
OK, now that said, you can also often just flip the butt fat side down and find a few of the larger glands just hanging out. They will look like pink chicken kidneys or light gray masses.
Sometimes you will find smaller darker glands that look like dark gray warts with some bloodiness around them.
And sometimes you won't find any glands at all! I think a big part of it depends on the specific pig and where the slaughterhouse makes their cuts. We've had plenty of butts with zero glands and others with a dozen.
I try to trim them out when I find them, but despite my boss's opinion, I can't help but feel that 5 grams of gland in a 25 lb. batch of sausage really won't make a difference. I'm sure if you are coarse grinding, and someone gets a big bite of gland, they may notice the armpit flavor, but in everyday grinding, I think you are fine. Just remove the big obvious glands as best practice and don't worry about removing all of them IMO.
Best of luck!