This is a page that you can place on your webserver that will allow you debug MTU problems remotely (as long as requests around 500 bytes are getting through, since that is the size of the html page itself + headers). You get the client machine to go to the web address on your server where you are hosting your copy, then have the person scroll down and read off the largest number that he sees. Each image is labelled with its own size, in bytes. If there is a fragmentation problem the larger images will not get through, and you can look at the number they give you as a clue to what size packets are getting through. Dropped larger packets could be either a problem on their end or yours, but if you are debugging it from a webserver perspective you are likely more worried about the problem being on your end.
MTU errors are particularly annoying because most of your standard network testing tools (ping, telnetting directly to the webserver on port 80, etc etc) mostly work with very small pieces of data and many administrators miss them. In particular, I ran into an issue when shoving data through an IPsec connection. Since encrypted packets cannot be fragmented, I ran into all manner of hell with real world applications but had all my normal tests report that the connections were fine. Using this eyechart as one of the standard tests in such a situation would have saved me some time.
online version of the mtu eyechart (on this server)
The MTU Eyechart, zipped up and ready to download.
Some further information about MTU discovery
mturoute - a console utility to trace the mtu between two points