In a logical, rational world, romantic relationships should follow a normal, natural progression. People meet, they date, they commit to each other, they gradually fall in love, they cleave in perpetual union, they die. Simple, yeah? Well, it certainly would be if only relationships existed in a logical, rational world. Sadly, they exist in an emotional and therefore chaotic world. This means that rather than following aforementioned progression, they follow a fickle and frightening trail right through the funhouse of feelings.
One of the most difficult and dangerous steps on this perilous path is trying to determine when you have moved from the "casual" phase into the "committed" phase of the relationship. What makes this so hard is that this change usually doesn't come with a ring. You don't typically change your address. No one grows anything in their belly for nine months. There really isn't any determining factor that can be used as a litmus test for when you have become "monogamous" or "exclusive."
So how do you know when have you made the change from a "casual" dating environment into the serious "committed" relationship?
Well, you could try the method I first employed which goes like this: Tell a person you have been seeing that you can't spend time with them because you have a date. If they throw something at you, congratulations, you are now in a committed relationship but, if you value your plates and your pets and would prefer they not be flung across the room, then you might want to consider a different tactic.
I have always been an overture kind of guy. I was trained as an economist, so I keep trying to cram logic and rationality into my relationships. It never really works. I tried asking where they see the relationship going and what status they think we should have. That's about as sexy and endearing as the average police interrogation, so I don't suggest it unless you've got a hyper-logical partner.
Since your partner is human, assume they are not hyper-logical.
The best way I've found to see if your relationship is moving into the serious phase is to look at their behavior and yours. People can lie to each other and themselves, but their actions will always betray them. Take a step back and see how the two of you are acting and I guarantee you will quickly see how each of you views the status of the relationship without the need for flying flatware or awkward cross-examination.
I use three primary factors for determining if we're casual or committed:
We spend our weekends together
You can't date around if you don't add it into your schedule. If you both are spending the bulk of your free time together, you're committed just because there isn't an opportunity to be anything else. If either party is consistently unavailable on weekends, they're still shopping.
We both have stuff at the other person's home
If you are comfortable with their things in your personal space and they are similarly content, then you've moved past casual. The more personal and important the items, the deeper entrenched the two of you are. If one of you never leaves anything in the other's home and is constantly giving stuff back, they are not ready to commit.
We've met the other persons' family and/or friends
It is a hassle to introduce flavor-of-the-month men or women to the people that are permanent in your life, so most people won't do it. If you are dating someone that you have acquainted with your friends and family, you intend to keep them around. If you have met theirs, they intend to hold on to you as well.
Naturally there is no sure-fire way to determine when the commitment line has been crossed. Plenty of people have been married for years and still aren't truly committed to their partner, so always tread carefully. I've merely found the above tricks generally act as a logical barometer for gauging where your otherwise illogical relationship stands.