Rule of thumb suggests 10 gallons of water per inch of shell. If your turtles are over 4 inches in length, you should be able to tell gender. Males will have long front claws and longer thicker tails. Females have shorter claws and tails.
If you have a female and she is ready to lay eggs, you must provide a nesting area. Laying eggs in the water is not an option and can cost the turtle it's life. I will add that when looking for a nesting area and there isn't one, the turtle is usually frantic and swimming against the glass, have you noticed this recently?
For the rest of the information regarding a proper set up, be sure to read through the main site
www.redearslider.com and read through the stickys on the forum pages. A proper setup is important to the overall health of your turtle and lack of can be one cause for a lack of appetite.