Out of square?

[noseoil]

Use two thumb tacks & two lengths of string to see if the platform is flat. Put a tack at each corner, then use the string to check the diagonals. If the strings touch in the middle, all is well. If not, the platform is more like a potato chip than a flat panel.

[KCStudly]

A variation on this string method is to use 3 equally thick gauge blocks. Clamp the string taut with a gauge block under each end, then slide the third block under the string at various points.

If the string is laying flat on the surface there is no way to judge a convex situation; the string will bow along with the arch and will still be laying on the surface. Whereas with the string raised above the surface the gap will get larger or smaller with variations.