You may be able to use the standard OS ping utility too?
Look for the options to set the DF bit 'do not fragment' and the size of the packet. For Windows, it's something like:
ping bbc.co.uk -f -l 1500
Just keep reducing it until you find the point where you don't receive the packet too big error, usually somewhere around 1472 for ICMP.
You should always get a response of some sort. If you see 'request timed out' you have problems
