I'm fairly confident you should be able to use the HealthyHostCount metric for your alarm, if you adjust it correctly. This assumes you're actually deploying a new EC2 host before draining off the old one. You can verify this by looking at your metrics and confirming your HealthyHostCount exceeds your DesiredHostCount for any period of time. If this isn't the case, update your question with the details, as that answer (detecting new service deployments vs new hosts) is a bit different.
Once you've verified your HealthyHostCount does in fact exceed your DesiredHostCount, you can setup a CloudWatch Alarm to just 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).