I'm no budgie expert but my guess is she's still very young. Maybe a few months because of the faint stripes on her forehead. Might be too young to tell for sure gender right now but I'm guessing female since it looks pink.
I'll let the experts correct me if I'm wrong though xD
Your budgie is very young and has not been through his/her first molt yet. The stripes on the top of the head will disappear and leave a solid yellow patch as he/she gets older. At this point, it is impossible to tell the gender.
It may be based on an old wive's tale, but I know a woman who can feel/hold a bird and "guess" gender ----she feels their abdomen/hip area and she has like a 95% batting average. AGAIN-- no science that I know of. I have just seen her do it repeatedly, and she doesn't claim to be psychic lol (so I know she is feeling for some sort of anatomical structure). IT IS EASY TO GET YOUR BIRD DNA SEXED...just saying, there my be other, less scientific way to (PERHAPS) guess...?