marker works best, get better lighting on the fakes the face and vertical lines are always showing on a real bill you have to get the right angle to see them. kinda hard to explain but if your holding it up in the light u expect to see the face hologram but if you put it down the face hologram will still show on the fakes.
on the new hundreds u can feel the blue strip with your nail, if your getting those bill people using them have some balls those are easy to spot.
20s rub your nail on jacksons coat or whatever that black thing is he is wearing you can feel the grooves. few simple easy tips. but the marker works best
https://fitsmallbusiness.com/how-to-detect-counterfeit-money/check this link out quick run down, #4 explains the hologram better than i can
also if you do find one hang it up so everyone can see your checking money its a good deterrent if its being used alot in your area.