Every time my friend, who lives about a five-hour drive away comes into town, he reminds me about how good he is at approaching and starting conversations with women the most simple and natural way. He is in his mid forties, but he is in great shape and he talks to women of all ages – from early twenties forward.
He almost always starts his conversations with women the same way. We would walk into a bar / restaurant together, and as soon as we pass by a group of women sitting at a table, he would point at one of their glasses or plates on the table and would ask with seemingly natural curiosity – “What is this?” or “What are you eating?” or “Is it good?” Then, he would follow up with “Where are you guys from?” as a natural transition to continue the conversation. If they act friendly and “help” the conversation by elaborating on where they are from and where they are headed that night, or even better – if they ask him questions about himself, then he knows that there is an interest and he can stick around and continue talking to the entire group.
Simple asking what a woman is drinking or what’s on her table, if the dish looks interesting and different is an extremely simple, natural and effective way to start a conversation with a woman. Among other things, it allows you to gauge a woman’s interest without being rejected. After all, you are not putting any pressure on the situation. You are not telling her about how beautiful she is and you are not asking her out. You are just making a normal and perfectly appropriate conversation. If she helps carry the conversation on after you ask her what she is drinking/eating, it likely means that she is single, available and might be interested. If, on the other hand, it appears like she is trying to cut the conversation short, it might mean that she is either not interested or a little shy and needs a little more time to open up to you and relax. By the time you ask her where she is from, how long she has been living in the area and whether they have been to that bar / restaurant before or any other kind of similar, simple conversation, you should be able to tell whether your presence is welcome or not, and whether you should stick around and talk to that girl for a little longer, or gracefully exit with “Nice to meet you, enjoy your evening”.
I like this because it's a genuine conversation starter rather than a silly pick up line.
Also, as you said, you can judge her attraction to you based on her willingness to continue the conversation.