Researchers told New Scientist magazine they found an area of thicker tissue among the women reporting orgasms.
But specialists warned there could be other reasons for this difference.
The existence of the G spot has remained controversial since the 1980s, when the term was coined as a way to explain why some women were able to achieve orgasm through vaginal stimulation, while others were not.
Some specialists claim that the term has led to over-anxiety among women who cannot reach satisfaction this way, and their partners.
The latest research, published in the Journal of Sexual Medicine, was carried out the Dr Emmanuele Jannini at the University of L’Aquila, and involved just 20 women.
Ultrasound was used to measure the size and shape of the tissue beyond the “front” wall of the vagina, often suggested as the location of the G spot.
In the nine women who reported being able to achieve vaginal orgasm, the tissues between the vagina and the urethra – which carries urine out of the body – were on average thicker than in the 11 women who could not reach orgasm this way.
Dr Jannini said: “For the first time, it is possible to determine by a simple, rapid and inexpensive method if a woman has got a G spot or not.”
However, Dr Tim Spector, from St Thomas’ Hospital in London, told New Scientist that the thicker tissue might actually be part of the clitoris – another extremely sensitive area.
Another suggestion was that, rather than being the cause of more orgasms, having these frequently might actually lead to better-developed musculature in this area.
Dr Petra Boynton, a sexual psychologist at University College London, said that an entire industry had grown up around the idea of a G spot, and it was unhelpful to label women unable to find theirs as “dysfunctional”.
She said: “We’re all different. Some women will have certain area within the vagina which will be very sensitive, and some won’t – but they won’t necessarily be in the area called the G spot.
“If a woman spends all her time worrying about whether she is normal, or has a G spot or not, she will focus on just one area, and ignore everything else.
“It’s telling people that there is a single, best way to have sex, which isn’t the right thing to do.”