Like other cloud providers, AWS provides free tier pricing to specifically allow users the ability to try out its services up to a certain limit.
Ignoring some services that are always free such as AWS IAM,
few considerations to note with the free tier model is that not all AWS services are covered under the free tier model.
For the ones that are covered under this pricing model, the free tier structure varies. For example,
a) there are some that are free for a longer duration (typically 12 months) up to a specified limit (e.g., AWS S3 is free for up to 5 GB usage with get/put request limits),
b) there are others that are always free services up to a certain limit usage (e.g., Azure functions with a million executions free per month) and
c) then there is another category that is free upto a certain shorter duration and/or credit limit. example, Amazon Redshift.
Once free tier exhausts, you are switched to pay-as-you-go pricing. This was often a source of confusion in practice for newbies who often do not have proper cost governance in place as it would be difficult to track when the transition to a paid pricing model has happened. Not that this information was entirely unavailable but it had to be figured out using the AWS billing and cost management service. Zero spend budget template is a handy way to know when you have moved out of free tier and the AWS free tier under the cost analysis section in Billing and cost management provides the current and forecasted usage of free tier eligible services.
AWS has now released the free tier API , which provides an alternate way to tackle this problem by providing details such as the actual consumption, the forecasted consumption for the billing period and the limit of the free tier usage. If the forecasted consumption is greater than the free tier limit, then it means that you will incur some cost if the service consumption continues at the same rate.
For now, this is available only using the SDK or CLI and any reporting, automation and alerting needs to be custom built.
A caveat - dont quote me on this as I haven't exhaustively used this API:). Like the free tier item in Billing and cost management, this API returns list of services that are eligible for some free tier usage in your current account. Ones that are not eligible, for example those that come under free tier pricing only for new users for specified duration (first 12 months after account creation as in case of AWS EC2) may not appear in the list based on how long this account has been in use. So, do not restrict monitoring of your AWS usage outside the free tier to only using these techniques.