To test and insure that the configured components work correctly you can use the tEcs-Library (recommended), alternatively use the instructions and snippets prepared for you in the section ERPConnect Services Runtime>Webservices>REST without tEcs “Table with REST” or another example.

Use this JSBin snippet to easily test your ECSCore and SAP Connection.

tEcs.testSapConnection({
    connection: {
        ecs: {
            core: true,
            coreApiKey: "E380FB721AFA4EB2A86CC624E4B6890C",
            //instance: "ec4",
            url: "https://ecscore.mydomain.com"
        }
    }
});

Another option is to build a query with browser extensions: POSTMAN (Chrome) or RESTClient (Firefox).

Here you must notice how the Authorization Header is set:

Authorization APIKEY

Base64 encoded key can be copied directly from the Management Site, you need to click on the generated API Key and select copy base64 value.