You may examine ascitic fluid both cytologically and biochemically but there is a good chance for uncomplicated tumor to show negative cytology. Otherwise, see the answers of Paolo Quilici and Olena Polyakova regarding intraoperative frozen sections and AFP/bHCG.
I fully agree with the opinion of the others wherein correlation, with clinical presentation, biochemistry (AFP-B-HCG), and ultrasonographic findings point out that this is most likely a germ cell tumour of the ovary. In an 18 year old woman, this is most likely a mature cystic teratoma involving both ovaries which is not uncommon. The findings of ascitis and omentoperitoneal thickening may perhaps suggest peritoneal irritation with, in my opinion, mesothelial reactive changes that is secondary to the compression effects of the huge ovarian masses, however of course, I cannot discount the possibility of a malignant teratoma with peritoneal metastasis, albeit this is rare in this age group. Exploratory laparotomy with ovarian resection (or frozen section), peritoneal implant biopsy, and ascitic fluid cytology, would be the management of choice. In my opinion a pre-operative ultrasound-guided percutaneous FNAB may yield not much of a helpful information as far as diagnostic determination between a benign vs malignant teratoma is concerned. A sample of the ascitic fluid of course may be collected for cytodiagnosis, but may well prove to be of limited value in this case. I would be most interested to follow up this case. Please update me with any latest findings. Thank you very much for sharing this interesting case.