I also had a lot of trouble when thinking about the "Are we on a relationship" and the infamous "I love you". I have a lot of problems related to understand what others are doing and the meaning of their actions, so what I did was to be very open and ask directly when I saw a good opportunity.
I asked my current partner 3 months after knowing her and 1 month after we started dating if she wanted a serious relationship with me. I said exactly like that because at the time I was heavily over thinking about the current state of our relationship.
This worked for me, BUT I also think that is very risky, since she said "I was quite surprised, but I like it". From what I asked and noticed from other couples, people tend to be more careful to say "I love you" and about "what about a relationship" to their partners, which I never understood before someone telling me that "It doesn't depends only on you. If you are ready to do something, before doing it, make sure that your partner is also ready, otherwise the outcome might not be what you expected and be ready for it."
In the end it depends on how you feel it is going, yours and the girl's current state to ask or not about it.
(You can take a look on my other questionmy other question to see if you relate to how I handled it as well)