When their lips are perked like they had just had bot ox injected into it.
Like this:
http://images.teamsugar.com/files/upl2/33/330044/20_2009/02ecb72f5a9df267_Collagen.jpg
If they are not ready and if they look like this, don't kiss them!!:
http://schema-root.org/region/americas/north_america/usa/government/officials/condoleezza_rice/condi.jpg
http://3.bp.blogspot.com/_zvCLjsWq_UU/SQGyP3Jcv8I/AAAAAAAAAgE/phqgiob2CRk/s400/rummy+pursed+lips.jpg
http://www.dailycognition.com/content/image/8/jocelyn-wildenstein2-tm.jpg
But seriously, there are no ways to tell if someone is ready to be kissed

You'll have to ask them if they want to kiss you

Good luck!!