The only reason Nick being a Grimm came to Juliette's mind after he had his tryst with Adalind was to explain why this happened. She figured that him being a Grimm was the reason this happened because Adalind wouldn't have entered their lives if he was a normal person. Juliette said as much in the car because she chalked this up with all the other bad stuff that happened to them over the last few years. She never thought Nick may have seen Adalind's woge because he's a Grimm.
To further explain myself, I'm saying Juliette questioned Nick on not knowing it was her because she didn't fully believe Nick could have no idea it wasn't her otherwise she wouldn't have questioned him, but she came around and saw the truth. She didn't question him based on him being a Grimm and him seeing below the surface to see Adalind's true form. Juliette wasn't thinking about Nick's supernatural abilities but his normal perceptive abilities.
I think the writers had the line from Monroe stating "no wonder Nick fell for it" to further show why Nick couldn't tell the difference.
To further explain myself, I'm saying Juliette questioned Nick on not knowing it was her because she didn't fully believe Nick could have no idea it wasn't her otherwise she wouldn't have questioned him, but she came around and saw the truth. She didn't question him based on him being a Grimm and him seeing below the surface to see Adalind's true form. Juliette wasn't thinking about Nick's supernatural abilities but his normal perceptive abilities.
I think the writers had the line from Monroe stating "no wonder Nick fell for it" to further show why Nick couldn't tell the difference.