[Lizard]

Select the last milestone. Open the task information inspector. Enter your deadline in the End constraint box. Then click 'Reset' up by "On specified date". Now your milestone should move earlier in the project, but with a horizontal line sticking out the right to the deadline.

Then if you do something (move tasks, add more tasks) that would push your milestone past the end constraint (your deadline), you will get a violation (a red stop sign with a hand) on the milestone, explaining that you've hit that constraint and offering some ways to fix it.