Tail observation is my preferred method of sexing snakes because it is the least invasive.

I have never tried sexing a Mandarin rat snake. Corn snakes and many others are fairly easy to sex by observing the tail, but it is 75-90% accurate if the snake is sexually mature and there are other snakes of the same species and known sex to compare with yours. Practice helps.

Most sources recommend looking at the tail's underside, with the tail on its back. I get better results looking at the tail from the side. (Try both and see which you prefer.) I mark the location of the vent and gently stretch the tail straight out. Then I find the midpoint between the vent and tail tip. Male tails generally taper slowly (because of the hemipenes) and are more than half as thick at the midpoint as at the vent. Female tails generally taper more rapidly and are less than half as thick at the midpoint as at the vent.

I could not tell what your snake's sex is. Because I could not identify the vent or the tail's midpoint. The snake is also younger than I prefer.

Good luck.