The Onshape Measure tool is available in Part Studios, for sketches and parts, and in Assemblies, for parts and assemblies, and for curves and surfaces as well. The Measure tool appears in the bottom right corner of the interface in Part Studios and Assemblies, (shown below, in a screenshot from Onshape's desktop application).

You can filter which measurements are shown by selecting a Measure type from the first dropdown.

Assembly Measure tool icon at lower right corner of the window Measure tool dialog Measure dialog with an example of a Part Studio

In addition, selecting any entity from the graphics area also automatically shows the pertinent measurements without having to explicitly open the Measure dialog.

You can also use the Measure tool on faces, edges, and vertices while in a Section view.