It depends on the situation, lol.
If it's my husband, then I'm definitely flirting on purpose. If it's a nice guy who happens to catch me off guard with his charisma, the flirting was probably accidental because it happened naturally until I realized what was happening.
There are other situations though, where a woman finds herself using flirting in one form or another. We might flirt if we feel it will help us get out of trouble, get that promotion or if it might cheer someone up.
As for when we give off "clues" (not signs) that we are flirting, each woman is different and it's hard to pin down an exact clues... I'll see if I can put it into a hub for you, cause it would take some explaining, lol.