A teacher can tell when they have successfully integrated technology in their classroom when the students are learning through that technology and the knowledge they are gaining pertains to their studies.
This technology should only be used if it fulfills the learning requirements and if there is someway to keep building on that knowledge. If technology can enrich the learning content of the students then it has done it's job.
Trial and error is the true way to see if the technology you have integrated is an appropriate one for the students but that is not ideal. You (the teacher) can tell if the technology is the best one by doing the learning yourself. Try the lesson for yourself and if it leaves out any parts there might be another technology that can teach it better.