Pretty much this is how a guy knows a girl is easy: when she gets crazy drunk and drapes herself all over him.
There's no "sex radar" guys are somehow blessed with. It is true that some people are simply better at reading signals than others, but it's not like we have a second sight that tells us exactly how much work it'll be to get into your pants.
Really, how much it takes is up to you.
I think that if you have a woman with low self esteem and a man who is a predator that they do have some sort of radar as does the woman. These sub-conscious behaviors would attract each other to continue that pattern till it is broken.
I am curious what easy means too? Does it mean you are desparate and will jump in bed with anyone? I guess that would be easy in the sense of not having strong boundaries for how you expect to be treated. Or does easy mean that you actually choose to have sex with many partners. If that is your choice and that is where you are in your life, go for it and have fun.
There is still the stereotype of men sleeping with a lot of women should be congratulated but a woman who sleeps with a lot of men is easy (and that is a nice word for it).