Females (sows) : The sows usually have a smooth swelling over their genital area. Sometimes, it can be bumpy looking, making you think it might be a male! If you gently part the genital opening, on a sow, a "Y" shaped opening should appear.
Males (boars) : Press gently just above the genital area. If it's a male, you should be able to make the penis slowly extrude. Don't be fooled by appearances. Sometimes, it doesn't look like there is a penis there at all! Especially in heavy, older males, the penis can be "tucked away" in folds of skin, looking entirely like a female! You should TRY to get the penis to ease out.
HOT TIP ! If it is a male, you can usually feel the inner part of the shaft (which is under the skin) in the same area--just above the genitals -- by very gently pressing and rubbing your finger over that area. You will feel a "ridge" just above the penis.
Also, if there is a pucker or protrusion of skin at the top of the genital area, then that is a good indication that there is a penis tucked away underneath. But, be sure to get it to extrude to confirm it.
Young males have a donut shape around their rectum where their testicles are.
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Article By : http://www.cavyspirit.com/sexing.htm