Cancer is quietly flirtatious, never forward. There will be roses and champagne over quiet meals, not wild cavorting at a beer bash.
And the Crab probably won't say exactly what's on his or her mind either. As such, it could be hard for a partner to know the score.
Cautious Cancer will never plunge headlong into a commitment, but if he or she starts to communicate nonstop via all media like a Gemini (and toss in words like "we" and "us"), or spend lots of time with you like a Leo, you've nabbed your Crab.