Surely one of the the easiest ways is if the circle is still a circle and not exploded is by right clicking on the circle (or highlighting it with the selection tool and then right clicking it) and then selecting the ‘find centre’ from the menu.
If you are aligning two circles of different diameters/radii you can use the line tool to draw a line at 90 deg to the face on each circle and select the line and the circle/cylinder to then move one to the other on the lines’ ends and snap the 2 line ends. The 2 circles will be aligned through the vertical centres and the lines and centre points deleted before moving the faces together or deleted using X-ray tool after the geometries merge.
Sound long winded but isn’t when you do it.