There's no real solid way to tell on the iPad but it's just a gut reaction. Speedtest will not show an increase most likely because it's not download speeds that are generally improved. I signed up for free, set up within minutes, and changed my dns server to theirs instead of my isp's, and I can tell a difference on any computer connected to my router at home.
Perhaps I just noticed a difference because my dns my ISP uses is just THAT bad, but it's worth a try.
OpenDNS | Internet Navigation And Security