You will know when you have one, believe me there:)
But, it is not always a 100% thing. I had them most of the time (85% +) when I was with my first love (my ex-husband too). It was because of the loving and trusting relationship that we shared. But, since we split, I haven't been able to let go enough to have them (about one or two in five years! *ACK*)
When you let go and are emotionally close AND when you are able to work with your partner to guide and share what is pleasurable to you, the likelyhood of the big O will be much more possible.