Your cockatiel has dark grey tail feathers and only two horizontal striped feathers does this mean it is a female?

No. not neccessarily. Male cockatiels usually are more vocal than females so start with that. If you still can not determine the sex. Look to see if he/she has lots of yellow around the face. If there is alot of yellow they are usually male. Then there the tail thing. Usually alot of the feathers will have the stripes but not all. If you still dont know, i suggest you take it to a vet and have it sexed.