Often times at first glance a board looks straight and the fact that it is actually bowed or has some warping isn’t always obvious. So the trick to knowing for sure, is to hold the board up towards your face, with the other end on the ground, and look at it at a downward angle (as shown in the below photo). This method will allow you to see if it is bowing at all.
This super-strong and simple-to-build workbench is may be the project you've been looking for a long time. You have to select some free workbench plans to create yourself a working table in your shed that after you can use it when you are working on your projects and maybe it can provide you some extra storage, depends upon which plan you are choosing to DIY.