To find your gender identity you must first know what gender means to you. There are two main factors that come into play with the concept of gender.
Your body -
Some people experience gender dysphoria. This is a distressing feeling some people experience where they feel wrong in their body or disconnected from their physical form due to their biological sex. This entails things like feeling as if you should not have breasts, or as if you are missing facial hair. Individuals with gender dysphoria often have triggering or negative feelings around things that make them aware of their biological sex such as urinating, menstruation, sex, erections, or hearing the pitch of your voice. Dysphoria does not always have to be feeling as though you should have different pieces, it can sometimes just be dysphoria against your current body. This is often associated with being gender-neutral transgender. It is also important to note that not all transgender individuals experience dysphoria or want full, or any, surgery.
Your prefered gender expression -
Gender expression is all about which gender norms you prefer to adhere to. This is an area I feel gets a little iffy because everyone has a different idea of how strict gender roles should be. So in some cultures being transgender would entail different things than in others because being male/female comes with wildly different norms and expectations in different cultures. This is why I say that you must first know what gender means to you before you can be sure of your own gender. You can be biologically male, want to do stereotypically feminine things like wear dresses and makeup and have long hair, and still be male because they gender identify male. Your gender expression does not define your gender identity.
The line between preferring the gender expression of a different (opposite or neutral) gender and being transgender is comfortability with yourself. If you are biologically female and prefer masculine gender expression but don’t mind female pronouns and just prefer that gender expression because it is what you like to do, don’t overthink it :). But if female pronouns make you cringe and when someone calls you dude or sir you find yourself getting a little excited, that may be something to think on.
What transgender really means -
To be transgender means, regardless of whether it is because of your body or your mind, that you are more comfortable having others know you, and thinking of yourself, as a different gender than your biological sex stereotypically correlates with. Regardless of the label of transgender, if you would be more comfortable in different clothes, with different hair, with a different name, different pronouns, or with a different body, it is your right to change, as much and as often as you want. That is not about gender, that is about your right to bodily autonomy, and no one should ever take that away from you.