About the only ways they can tell you're using a parser is if you either

a) post video of yourself using one, which they will usually ignore unless given a reason not to, but since they can hit you on the technicality if they want to, I do not suggest doing this

or

b) you straight up admit in-game that you are using a parser, as in the sentence "I'm looking at your numbers on my parser and your DPS is shit bro." This is an excellent way to commit account seppuku with a GM serving as your kaishakunin and I extremely suggest not doing this.