Check for smooth lines or a single straight line that runs deep into the marble.
Marble on microscope.
Rocks under the microscope.
If the marble is completely smooth it was machine made and has no pontil marks.
These are photomicrographs very thin slices of rock seen in plane polarised light or between crossed polarisers when the colours seen are produced by interference of light.
They are shear lines caused by machines and are not true pontil marks.
Lines like this as well as smooth indents or bubbles do not indicate pontil marks found on handmade marbles.
Some common rock types as seen under the microscope.