Possible to make a checkboxField check mark bold for disabled property?

As mike said it is not possible but we can play with icons "check-square" this can help

To expand on the above, you would show a!richTextDisplayField() with a!richTextIcon() instead, when the a!checkboxField() would otherwise be disabled, such as:

a!localVariables(
  local!disabled: true,
  {
    a!checkboxField(
      label: "Possible Duplicate",
      choiceLabels: {""},
      choiceValues: {true},
      value: true,
      showWhen: not(local!disabled)
    ),
    a!richTextDisplayField(
      label: "Possible Duplicate",
      showWhen: local!disabled,
      value: a!richTextIcon(
        icon: "check-square-o",
        size: "MEDIUM"
      )
    )
  }
)

To expand on the above, you would show a!richTextDisplayField() with a!richTextIcon() instead, when the a!checkboxField() would otherwise be disabled, such as:

a!localVariables(
  local!disabled: true,
  {
    a!checkboxField(
      label: "Possible Duplicate",
      choiceLabels: {""},
      choiceValues: {true},
      value: true,
      showWhen: not(local!disabled)
    ),
    a!richTextDisplayField(
      label: "Possible Duplicate",
      showWhen: local!disabled,
      value: a!richTextIcon(
        icon: "check-square-o",
        size: "MEDIUM"
      )
    )
  }
)

Thanks Chris.

Is there a list of icons amiable for displaying?  

docs.appian.com/.../ux_styled_icons.html

Yep, Mike has you covered there.  Also as a handy feature, if you are in expression mode the rule definitions typically have links you can use for quick access to the available icons: