You need to get one of those wattage meters. The cheap one is "kill a watt". Just google it or search at amazon, It should be under $20. Then try charging your iPad when the battery is about half full. On good cable you should get from 10 to 12 Watts. If you use iPhone cable, you will only get 5-7 Watts. I heard newer iPhone cable is rated the same as iPad cable. Some cheap cable from ebay definitely won't go over 7 watts; I have a couple of those.
The good ones are obviously Apple's cables but some are fake and look just like Apple's cable. But Apple's iPhone cable is also rated lower and not for iPad. So for iPad, use Apple's iPad cable. For iPhone, it doesn't matter since it doesn't need more than 10 watt; any cables, real or fake, can deliver 5 Watt.