Well, first look to see if your mommy tied your shoes for you. Next, check to make sure your undies are on correctly---yellow stain in front, brown in back. Oh wait, my goof----you're probably already waaaaay befuddled reading all this!
I was in the same boat at one point-- I hit the weekly hours to be auto-promoted to full time and didn't actually notice until I got my insurance paperwork.
Easiest quick way is to look at the schedule sheets. All full time employees are at the top, from most seniority to least. Then it starts with the most senior part time employee. So if you're above someone you KNOW started after you, or you're suddenly in a different spot than you were last week, you're full time.