Watch for certain expressions. Smiling is one of the clearest signs of attraction. Make sure you know how to decipher a forced smile from a real one. You can tell fake smiles from real ones because the smile doesn't reach all the way o their eyes. Genuine smile usually result in tiny crinkles around a person's eyes ( crow's feet ). When people fake smile , you won't see the wrinkles.
Raising the eyebrows has also been observed as a sign of flirt.