The standard response is that if you ever thinking about your gender this way, you are trans. As it turns out cis gendered folk don't do this generally speaking. But there's a whole spectrum, you'll have to decide where you fit on it, I just recently finished that, only took me about 10 years after first realizing I was probably trans, but now I can admit to myself that I should've been born female, nothing short of HRT will help me, haven't decided on SRS yet, leaning towards it, but I've got a good few years before I need to make that decision.
What helped me come to this conclusion was to do things I wanted to do without taking into account the little voice screaming "it's not OK for men to do this"
What I found was, even though I was shopping in the men's department, the shirt I bought was very feminine, and I look not female exactly with it on, but certainly not just male. I'll often instinctively take a feminine posture, I really wanted longer hair, so I grew it out, etc. This will probably not work exactly the same for everyone, requires a certain level of mental discipline, I wasn't seeking out the feminine, rather just doing what I REALLY wanted to do, and as it turns out, I'm pretty universally drawn to the feminine.
That isn't to say that I don't enjoy some stereotypical male things, like competitive online games, but plenty of girls do too. It also depends what kind of girl you are, you might be a tomboy, in which case this experiment might produce confusing results, lol.
At the end of the day it'll be a personal process, but there is one common thread in all trans people, once they have these thoughts, they never go away on their own, some can keep them under control with cross-dressing or drag performance, others like me need to be seen as female 24/7 to be really happy... And it's completely normal to feel like that, for 15 years I told myself I didn't really want to transition, but I was lying.