If issues are mapped to project milestones, you can see that also in the issue itself. By clicking on the project context button on top, a panel opens which shows the project context (milestone) where the issue is mapped to.

Project context button and milestone information in an issue

If the issue isn’t mapped to any milestone yet, you can add one by clicking on the blue button. If a project context exists, it can easily be edited or removed by clicking on the respective icon.