Create a custom renderer. Extend the DefaultTableCellRenderer and add your own code to display whatever you want. It could be a custom Icon or if the "checkmark" is a printable character than you can just set the renderer text to the appropriate character.
Read the JTable API and you will find a link to the Swing tutorial on "How to Use Tables" which will give more information about renderers.
If you need more help post your SSCCE showing the problems you are having creating the renderer.
Edit:
The tutorial shows how to add a custom renderer for a given class but it doesn't show how to add a custom renderer for a specific column. You would use:
table.getColumnModel().getColumn(...).setCellRenderer(...);
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