Iterate the response headers and check the Content-Type

validate-content-type-header.js

function validate(response) {
    var headers = response.getHeaders();
 
    console.log("The response has " + headers.size() + " headers");
 
    for (var i = 0; i < headers.size(); i++) {
        var header = headers.get(i);
 
        if (header.name == "Content-Type") {
            if (header.value == "application/json") {
                return true; // validation succeeds!
            }
        }
    }
 
    return false;
}