Right click on the web page, click on "Inspect the element" or "Inspect", then go to the "Network" tab, reload the page and look at the "status" column. You can find the different http codes in the ref-api documentation too.
For the firefox browser, there is a color code for the http codes, green or white when it is normal and red when it is an error.
The different http codes :
Code 200 or 204: Ok
This code means that the request has been correctly executed.
Code 307 : "CHECKENROLLMENT"
This code appears when a transaction is in 3DS and the 3DS validation is missing.
Code 400: BAD REQUEST
This code means that the request sent is not correct, a parameter or a rule is not correct.
Code 401: UNAUTHORIZED
This code means that the authentication parameters are missing in the request.
Code 402: PAYMENT_REQUIRED
This code means that the payment authorisation was denied.
Code 403: FORBIDDEN
This code means that there is a bad authentication and entitlement or authorisation problem. This means that you are not authorised to make this request.
Code 404: NOT FOUND
This code means that the request was not found or does not exist.
Code 500: INTERNAL SERVER ERROR
This code means that there is a server error. If you have this error, contact technical support quickly with the items advised in "Technical support request".