Issue View
When issues are linked to project milestones, this information is also visible within the issue itself. On the right sidebar, there is a section labeled "PM." When this section is collapsed, it indicates whether the issue is associated with a milestone or not.

Project Milestones information in an issue
If the section is expanded, you can see more details.

Expanded Project Milestone section in an issue
If an issue is not associated with a milestone, you can easily map it to one directly within the issue itself. Additionally, you have the option to edit the linked milestone or remove it altogether.