Thoughts on my apartment? (I'm new to learning about mold)

I don't usually jump into asking questions - I like to lurk and research on my own first - but I just stumbled on this forum tonight and, well, I'm moving into the place in question next week! I'm moving in with my best friend to a second floor apartment (next to a river, if that matters.) To understand the situation, my boyfriend and I are jumping on the lease late (we planned to do that). My friend has been in central America for the last month, so I've sent her an e-mail asking about the apartment, but I don't know when she'll respond. The month before she left she barely spent any time in the apartment at all, so she wouldn't be displaying any mold poisoning symptoms if there is a problem.

Anyway, the room I'm moving into had water damage at some point - the landlords told my friend when she moved in that the damage was fixed, and the wall looks fine. However, she said once that the room smells a little musty/mildewy if you leave the door closed. Honestly, I thought nothing of it and I've never actually been there when the door HASN'T been open. I am moving some of my things in this week so I will try to check how it smells when the door's closed, and if it is bad... what do I do? Isn't mustiness a sign of possible mold growth? My brother had a run-in with mold last year that left him sick for months, so I'm very concerned (and maybe slightly paranoid) about this.

So, I guess my main question is... how do I check if there is mold? would the kits you can buy actually work in this situation? Is there something specific I should ask my landlords...? I'm nervous to do anything, seeing as this is my first time living away from home. If I can get some guidance or reassurance from someone I would really appreciate it. Tweet