If you are doing a site here and there then the suggestion from Acorn above is a good start, but if you are doing this for clients then you need to have an iOS phone, an iPad and and Android phone and Tablet at the very least. There is no substitute for the real thing (with all of their own little foibles). You do not have to test every single break point and every single device but the four I have mentioned plus obviously a desktop/laptop work great.