The Dustball Plagiarism Checker is a brilliant tool for doing this. On the first page you copy and paste student work into the box. It seems to carry any amount of text and even if the original document has pictures it works around them.
After this, hit the 'check' button and a new page will pop up with a selection of sentences and the verdict - 'ok' or 'suspected plagiarism'
Finally, and this is my favourite bit, by clicking on the 'possible plagiarism' bit you get taken to a google page showing the websites. You are then able to present the sudent with the website that the text was taken from. It's also worth checking these as sometimes there are a LOT of hits with the same sentence - in this case, it's simply that the topic means a student is likely to write that combination of words. If there's only one hit returned, and it matches a lot of the work, then it's likely to be plagiarism.
* A quick note: All of the websites I write about are genuinely ones I use in my classroom - they are not sponsored links or anything like that. I write about them mostly so I have a quick place to get the link from. This does mean that some weeks I won't write about websites because I don't have any new ones.