You can wait until the tadpole is an adult frog to determine the sex in a far easier manner. For example, female frogs grow larger than male frogs, and male frogs will generally be observed calling to females.
After most species of frogs hatch from eggs, they enter the tadpole stage. This stage lasts six to nine weeks, at which point the tadpole will begin to grow arms and legs on its way to becoming a frog.
Determining the sex (also called "sexing") of a tadpole must be done surgically, using a very powerful microscope. This is typically done by scientists in a laboratory for environmental research to learn how some species of tadpoles can change their gender due to chemicals present in the environment, such as the pesticide DDT.
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Using the scalpel, cut one-micron thick slices of a dead tadpole's mid section, toward the base of the tail. This is where the gonads will be, and these are what are needed to identify the gender of the tadpole.
Mount the slices on glass slides, and examine them under an electron microscope.
If the gonads are solid, dark circles, the tadpole is a male. If the gonads are ring-shaped, they are ovaries and the tadpole is a female.