I am a dog person, but was adopted by a cat many years ago. Similar to your situation, he was very skinny, walked into the house, and basically stayed there for the next 15 years or so. I wormed him as a matter of course, and de fleaed him, but on closer inspection found him to be red raw underneath, and ended up taking him to the vets, and he had a flea allergy. By coincidence I found the owner, who said that he ran off never to return when she had a dog, strange thing was I had a dog which was the same breed. She/He could have worms, and could be hungry, if it were me, I would take the cat in and ask around to see if it belongs to anyone, but in the meantime, do whatever is needed - flea treatment and worming products can be bought cheaply on line
http://www.vetuk.co.uk/index.php?main_page=ind ex&cPath=3&zenid=0d5dc82b85736d17b6966042ab51b 748
http://www.vetuk.co.uk/index.php?main_page=ind ex&cPath=17
if you do decide to de flea and worm, the products in the pet shop are a waste of money.
Not much good in the sexing department though, sorry, maybe someone else can help? If it was an unneutered male it would probably smell, and an unspeyed female would need to be speyed, but if you are taking in a stray cat the cats protection league would pay for veterinary costs.
http://www.cats.org.uk/
Age, difficult, is she lively, kitten like, what are her teeth like?