Is it on AWS?

Yes, www.expander.pl (34.250.182.142) is apparently on AWS.

Region: eu-west-1
Service: EC2
CIDR: 34.248.0.0/13

Discover whether something on the Web is on AWS.

This works by looking up IP addresses in the IP Address Ranges JSON file published regularly by AWS.

It’s perfectly possible that the site you looked up is using a content delivery network other than CloudFront, or has its front page on its own computers but most of its business logic on AWS (or vice-versa). So the picture isn’t as simple as this page might make it look.

This site makes use of several AWS services in an interesting way; details are in this blog. Also, the source code is available.