How did he knew about female omegas

Situ Jie looked at the omega man who was kneeling on the floor, she turned his face left and right before she nodded in excitement as her hand continued to hold the omega man's face. Her nails delicately touched the skin of the man since she was afraid that she will scar this wonderful gift that she has gotten from her sister. 

"It is a wonderful substitute sister Zhilan! I cannot thank you enough for handing me this gift, I will surely take care of this gift with all my heart. Don't worry there won't be even one scar on the face of this man," Situ Jie smiled at her sister as she looked at the pretty omega who sat next to her with a polite expression on his face not even daring to raise his head which made her mood to inflate as she nodded in appreciation——this was how a servant should act.