Last year (we were at 7 parties between Halloween and Christmas) the CM's were very strict about enforcing this. Just a few things we noticed.
From 6pm, announcements were made through the park that the Magic Kingdom would be closing at 7pm to day guests. This announcement was made several times up to 7pm. At 7pm, an announcement was made that the party had commenced and those without tickets needed to head for the exit.
CM's were placed around the Hub. Once it was 6:45pm, no one was allowed past the Hub from Main St if they didn't have wrist bands.
CM's were placed at the entrance of each land, checking bands from 7pm. No band, you were asked to leave.
CM's were checking for bands as people started to line up for the parade. No band, you were asked to leave or escorted out.
CM's were checking for bands just before Hallo/Holiday Wishes. No band, you were asked to leave.
CM's were places at the entrance of the main gift shops and counter service. No band, you weren't allowed in. At 6:45pm, CM's were telling people inside the stores that if they didn't have a party ticket, they needed to complete transactions from 7pm otherwise they weren't allowed to put them through. After 7pm, no one with a ticket was served. You couldn't buy party merchandise without a ticket (some stores had it out before 7pm)
CM's were escorting people out of the park after their ADR's if they didn't have a party band. We saw this from BOG, Crystal Palace and CRT.
CM's were placed at each bathroom after 7pm. If you didn't have a band, you weren't allowed in. You could access one bathroom without a band, which was on Main St near Guest Services. After this, you were escorted out of the park.
CM's were checking bands at all rides. You weren't allowed to line up or ride without a band.
CM's were checking character lines from 6:45pm for anyone without a band. We saw someone was asked to leave the Merida line at
MNSSHP at 6:50pm as she didn't come out until after 7pm.
They were very strict about it (as they should be)