Take a long, hard look at the time you spend together. Are you two just going through the motions? Is the conversation strained or routine? A good indicator that she needs space is when time spent together feels more like a habit, a ritual instead of something to anticipate. For example, when she asks you how your day was, do you get the sense she's asking because she's genuinely interested, or is it more out of habit or obligation? If the latter, don't confront her just yet; simply take in the data and begin considering the possibility of her needing more room in the relationship.
Pay attention to returned calls, emails or texts. There's a lot to be gleaned from both how often she responds and what she says when she does respond. If she leaves your messages unanswered or is cold and abrupt when she does respond, then her perception may be that you are contacting her too much; she feels confined. Granted, that presumably isn't your intention, but consider backing off a bit if the aforementioned strained communication is becoming the norm.
Ask yourself if she is spending more time with friends. Obviously, there's nothing wrong with a healthy social life outside of your relationship, but if the balance tips in such a way that her friends are getting more face time than you, it's a warning sign that she may need more space.
Stop initiating communication and contact. Don't become estranged, mind you, but as a test, see if she actually takes the lead on all forms of relationship activity, i.e., phone calls, emails, text messaging, intimacy, and date nights. If it turns out that you are doing all of the initiating and the relationship grinds to a virtual standstill, then you should introduce the topic and offer more space.