Placement Group Resources

Unfortunately, AWS does not provide a way to pre-check or guarantee the availability of specific instance types or resources within a placement group before you actually try to launch them. The availability of resources in a placement group depends on the current capacity in the Availability Zone and can change dynamically.

When you launch instances into a placement group, especially a cluster placement group, it's recommended to use the same instance type for all instances to increase the likelihood of having the required capacity available. However, even this doesn't guarantee success.

The best approach is to actually attempt to launch the desired instances into the placement group. If the launch fails due to insufficient capacity, you may need to try again later or consider using different instance types that are compatible with your workload requirements.

It's also worth noting that you can use On-Demand Capacity Reservations to reserve capacity explicitly in a cluster placement group. This can help ensure that the capacity you need is available when you launch instances. However, keep in mind that zonal Reserved Instances cannot reserve capacity explicitly in a placement group.

If you have specific capacity needs, it's advisable to work with AWS Support or your account team to discuss your requirements and explore potential solutions or recommendations for your use case.
Sources
Placement strategies for your placement groups - Amazon Elastic Compute Cloud
Placement groups for your Amazon EC2 instances - Amazon Elastic Compute Cloud