Maybe there isn't a word for this. After all when we tell someone something, or when we state something, it is understood that we're not lying unless otherwise noted. Merriam-Webster offers "enlighten" as an antonym for deceive, but I am not sure that that is a precise antonym. To me "enlighten" implies leading someone from ignorance into knowledge (sounds pretty noble that way!), again with an implicit veracity.

Perhaps the Ferengi on Star Trek the Next Generation would have a special word for telling the truth.