Even on the small thing, you ask him if he gets defensive or talks more than required, it means he has a guilty conscience.
If he is not romantic anymore or does not give you enough time in bed, that means his mind is diverting.
If he is late and you ask him why, and he makes too many excuses and overexplains, it means there is someone else.
If he was okay before for using his phone but now he avoids letting you use it, surely there is something fishy.
If all of a sudden his dressing style has changed or is wearing more attractive clothes, then you need to beware.
A man usually tells his close friends first about someone new and if they are acting strange towards you, this is a red flag.
If he asks more about boundaries and privacy, then someone else is more important to him.
If you are talking about something and he says, that that girl also does the same or anything like that, then she is in her mind.