Based on the tail feathers, it's really difficult to tell whether there is a beginning of a curl (suggesting a male) or just an out-of-place feather due to camera angle (meaning hen).
If the bird is 3 months old and has developed its quack, you might be able to tell drake from hen based on the pitch of the quack. Drakes have a sort of raspy, reluctant quack that's relatively deep in pitch. In young drakes, the sound that they make may seem rather hoarse, like they have a dry throat. Hens have a raucous, louder, often squeal-like quack when you're holding them. Young hens may have a sudden "hiccup" of adult hen noises between the more duckling-associated screams/loud peeping.
From the size of your Pekin, I would guess that Ms. Nibbles is more than capable of producing some sort of quack. Whether the quack denotes a drake or hen will be the deciding factor.
Best of luck!
