Tool - Spirit Level
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Stabila is generally regarded as the most accurate brand. They are expensive, but getting accurate leveling is important, so it's worth spending the money. You should always use the longest spirit level that will fit in the space you have available. That means that you are going to need a medium length one and a long one. Sometimes you can get a bundle deal that includes two different sizes. Here's the pair I brought for about $90:
http://www.amazon.com/gp/product/B000E7SV2I/ref=ox_ya_oh_product .