How do I know if I'm considered having "supported myself" if I am a college student living in my own apartment?

Support would be the expenses for your necessities, such as food, housing, clothing, medical care, education, utilities, etc...

You would have to pay for more that half of your support in order for someone else, such as your parents, to not be able to support you and thereby not be able to claim you as a dependent on their tax return. You can learn more from this TurboTax article.

The answer is it depends. It depends on more info.  How do you support yourself (working, student loans, scholarships, 529 plan, savings & investments)? is your apartment only your housing while you are "away at school" or have you permanently moved out of your parents home. Going off to school almost never qualifies as moving out.  What are you trying to accomplish by claiming you are not a dependent.  

With the tax law change, effective 2018, most students will get the same refund whether they claim themselves or not. The personal exemption has been eliminated and the standard deduction increased. However, you only qualify for an education credit, if you can not be a dependent.

Be aware that there's a new urban myth among college students that says they can get a $1000 from the government just for filing a tax form. For most of them, they simply aren't eligible. A full time unmarried student, under age 24, even if you don't qualify as a dependent, is only eligible for the refundable portion of the American Opportunity Credit if he supports himself by working. You cannot be supporting yourself on parental support, 529 plans or student loans & grants. It is usually best if the parent claims that credit. 

You cannot claim the (up to) $1000 refundable credit if you are, or can be, claimed as a dependent by someone else.

Reference: Line 7 instructions for form 8863.

https://www.irs.gov/instructions/i8863#en_US_2024_publink53002gd0e674