First, make sure you truly love the person. Do you know them to their true heart and soul and are you willing to let them know you in that way? That speaks to the level of security and comfort that your relationship provides to you. Are you compatible in everything from the routine everyday things to your views on politics, religion, raising kids, how to handle finances, etc.? That doesn't mean that the other person is just like you or even similar, just that the two of you can find some common ground and can show respect for each other's viewpoints- there's nothing wrong with a good debate or even an argument. Those things are healthy as long as they're conducted within certain boundaries (no name calling, nothing physical, no bringing up things from the past that should've been taken care of already, etc.). Are you friends? I mean, would you still want to know this person and have them in your life if you weren't involved romantically and sexually? Don't expect your significant other to "complete you"- you better come to the table mostly formed (as far as your personality is concerned) and you better not accept anything less from him/her. Does the person really know
you- not any of the roles you play or the airs you put on- just you and are you appreciated on that level? If you have to play a certain role or be any way other than what you truly are to keep the person, the person doesn't really love you because they don't know the real you. Finally, make sure that there's a spark of chemistry between you- I don't just mean sexually, I mean on every level intellectually, emotionally, etc. . . . although the sex part is probably the most important (especially when it comes to making up after those arguments that I mentioned)
