We are happy to offer paper proofs but a small print won’t reveal the most common issues we see with poster files. To “proof” a file yourself, export your PDF from the program it was authored in, then open it in a pdf viewer (Preview on the Mac and Adobe Reader on Windows) and view at 100%. First of all, if it’s sized correctly, it will look very large on your monitor and you’ll have to scroll around. If that’s not the case your image is too small.
If it is large, besides typos and correct data, you’re mainly looking for pixelation in your placed graphics.