What are the different ways to measure the spatial curvature of the universe?

Question

Just what the question asks. Assuming the Friedmann-Rovertson-Walker (FRW) metric, what measurements can be performed to determine the spatial curvature of the universe.

Answer 1

In the standard model of cosmology, there is a critical value of mass-energy density:

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for which the spatial curvature of the universe would be zero.

Values of density under or over this critical density would mean a positive or negative spatial curvature.

So by measuring the value of this density one can determine the spatial curvature, and current measures are pointing towards a flat and expanding universe.