On Edukado.net you can read about the KER exam that is offered. Since 2008, 1800 people have taken and succeeded in the exams at these levels: 1 at A1, 510 at B1, 543 at B2, 746 at C1.
The certificate itself is in four languages (Esperanto, angla, germana kaj franca). The testing is accredited by ALTE, The Association of Language Testers in Europe, an organization set up by the universities of Cambridge and Salamanca.
You can't take this online, as far as I've seen it. You can ask for a local organization to organize the exam for you, I think.
To prepare yourself for the exam: http://edukado.net/ekzamenoj/ker/specimenoj