well it varies from girl to girl. here are a few, but please note most of these aren's sure signs, just indicators.
If when they talk to you they smile alot, for example laughing at things that arent funny, or just smiling at everything you say. or if you happen to catch their eye across a room and they return the look and smile.
if they tend to stand close to your when talking, like if they inavde your personal space and stand quite close to you
lots of hugs and being over nice can be a girls way of telling you she wants you to know she likes you
if they compliment you lots, that can be a sign of them flirting with you. if they arent so good at flirting it will be less subtle
Some girls fiddle with their hair while flirting
if your good at reading body language it can be obvious. if your not then hope this helps =]
also these signs may sounds obvious, but girls are nothing if not subtle, they happen to be the masters of confusing you!