Yeap, sounds to me like someone who's having issues balancing her pot and her work/school-life. If you can't do it, DON'T smoke it!
If we're talking serious drugs, you might be able to tell just by being around her or talking to her. Not always the case though, like our ex-roommate that robbed our house and turned into a prostitute (literally), we couldn't really tell she was doing H at first. We think she was snorting it to begin with instead of the traditional injection method, so there weren't any tracks until later. But it only took us about a month to figure it out. If she seems overall normal and the only odd behavior is that she can't manage her money, then it's probably not hard drugs. If she's acting really shady, doesn't "look right", is far more secretive than she used to be, gets standoffish (out of proportion) when questioned, has constant new "friends" that are alternating, she might have a problem.
But yeah, either she's like I was in my early college days and can't say no to a beer and/or joint, or she's like my late cousin who tried so hard to get everyone to like her she was always blowing money on others to impress them, OR she's like my bff Lisa who has a penchant for expensive things and has to have them "NOW" (she's recovered now and is pretty frugal these days).
Either way, if you don't cater to her, she'll probably get it after a minute out of necessity. If she's a pothead and/or partygoer, she'll either figure out how to balance her buzz with her budget or quit outright. If she's just spoiled and either blowing money on expensive things for either herself or her friends, she just needs to grow the fuck up!
No matter what, plant your foot down and keep it there.