Here are some of the observations you might want to consider to know if someone is cheating on you through text.
Always busy texting but immediately turns off his/her phone when you are near.
Very protective of his/her phone.
Keeps on changing his/her screen lock passcode.
He/She is just waiting for you to fall asleep at night to freely text message his/her other man/woman.
He/She can either be sweeter than before to you or always irritated at you.