Internet ridiculously slow -- me, someone else, or my ISP?

[D1sc1pl3 0f Mal1c3]

Is there some diagnostic tool that can determine this? I live in a house with a bunch of other people, and me internet has been running really goddamn slowly on and off for a while now. I want to find out if someone is soaking up all the bandwidth, I've got a ton of trojans or something, or SBC just plain sucks.