Rowanwitch is right, diamonds scratch glass. Diamonds also get a very fine layer of grease on their surface after a few days of wear without cleaning, experts can spot this immediately. If you have a jeweller's lens you should also spot tiny black inclusions inside the stone if it is real and not the very best quality. Diamonds are almost always set in gold, white gold or platinum so look for a gold or platinum hallmark on the metal. If the ring is silver (slight blackening visible in the crevices of the silver metal) it is unlikely that you have a diamond.
Other than taking the ring to a jeweller you could compare it with other stones that you know are not diamonds such as glass (paste), white sapphire, quartz, white topaz and zircons - ask your friends for samples.