So I had a fiddle and found a brilliant script by Remy Sharp that uses the File API in HTML5, with a bit of fiddling I managed to create the "Tiling Texture Tester" - the name may need work but the tool is really useful. Read on below to find out how to use it.
You simply save your texture file to a PNG/JPG/GIF, then drag and drop it into the dashed area, the page will then automatically tile it so you can see how well it tiles and if the repeat on the tile is too obvious.
The really handy thing is this page requires no extra scripts/downloads/plug-ins for it to run, so all you need to do is: go to the link here, and either use the site live, or save a copy of the page to your desktop and run it from there.
If you still don't understand what I'm talking about - save the two images below and try them in the tool, what you should see is the left image repeats nicely with no obvious seams, where as the image on the right has quite obvious seams that need fixing to use it as a tile-able texture or pattern.
If you think of any improvements or want to improve the tool yourself, go for it, but send me a link I would be interested to see what people come up with.