You could probably use the HealthyHostCount metric to configure your use case. This would alarm you if configured correctly.
This assumes you're actually deploying a new EC2 host before draining off the old one. You can verify this by watching your metrics and confirming your HealthyHostCount exceeds your DesiredHostCount for any period of your time.
Once you have done the verification and when HealthyHostCount does, in fact, exceed your DesiredHostCount, you'll set up a CloudWatch Alarm to only detect any maximum > [DesiredCount] for 1 out of 1 data points for any 1 minute period.
The period you set here should be small enough so as to not overlap with successive deployments (since the max won't change within a given period if you've deployed multiple times).