You can do the following:
In the navigation pane, choose Instances. In the top right corner, choose the Show/Hide icon, and then select Lifecycle. For each instance, Lifecycle is either normal, spot, or scheduled.
Alternatively run this CLI command
aws ec2 describe-spot-instance-requests \ --query "SpotInstanceRequests[*].{ID:InstanceId}"
Source: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/spot-requests.html#using-spot-instances-running