I don't know about the fed level, but in my limited experience the haircut thing is not really accurate. I feel like looking at haircut and shoes comes from another and much more primitive era of law enforcement. A lot of undercover people work long-term as undercover people, and they have deliberately sort of "fellow kids" hair and facial hair.
Vibes and how well you know the person are more reliable than any specific checklist. If you wanted the checklist though, this is not a bad one.
One additional thing I've noticed as a tell is that everything is polished. Whatever type of "street" clothes they want to put on, the shirt tends to be neat and spotless, everything is clean, the facial hair is groomed. I imagine it varies by the skill level of the person, but in general cops don't like to be sloppy. Even their "sloppiness" costume will still be neat and presentable.