Not sure why this question was negatively rated. Its not easy to determine this without specialist networking equipment, but you can get some way there through deductive reasoning.
The first thing would be to look at the ports on your switch/router and look at how often the lights blink. The more often, the more data. (This is rather crude an inaccurate, but its simple to look at). If the light is flashing hard when very little is being done on the computer thats most likely the cause of your problem.
It could also be your connection - if you are using ADSL, the upload speed is a LOT slower then the download speed and this could be causing you issues. If someone is uploading large files, this could impact your speed.
It is also possible the problem is simply the server you are connecting to is getting overloaded at peak times (although this is unlikely). Using MTR (or WinMTR or similar) you should be able to see where the latency is coming in, which could show if its a LAN issue (latency spikes on the first hop), a constrained connection to the Internet (second hop), or a problem with your provider.