Is there any way of viewing, in AWS, what ips in a subnet have been allocated?

In EC2 console go to Network interfaces view down in the left hand side column. It will show all the network interfaces allocated not only to EC2s but also to Fargate, RDS, VPC Lambdas, NAT Gateways, etc.

Note also that there is a couple of IPs reserved in each VPC and Subnet - IGW, AWS DNS, etc. IIRC it’s the first 5 IPs that are reserved. These will not show in the list above.

Hope that helps :)