Try searching. This has been discussed many times. I personally give the ooth more than enough time. If nothing hatches I carefully slice off the front of the ooth until I encounter the first layer of eggs. If they are dry or dry and black they won't hatch. If they are yellow and full of liquid give it more time. If there are little mantids then it will hatch soon.
Like Rick said, time is the best way to see if the ooth is viable. I say that because it's the least invasive thing to do. But if you really must find out now so you dont wait the next couple months wondering, then check out this thread: http://mantidforum.com/forum/viewtopic.php?t=3753