You can use a Point plugin. Get one from either the Extension Warehouse, or the Sketchucation plugin store.
Start the Point tool.
Hover the mouse pointer over the edge of the circle to see an inference point at its centre, move the mouse to that centre, then when the centre inference highlights, click on it to draw a guide point there.
You may also be able to do it with the native Tape Measure tool, but a Point plugin is easier.