Usually, for science and engineering fields, all PhDs are awarded a funding package that pays for the cost of tuition and provides a salary. Some schools award it as a tuition waiver, some as a fellowship, and some just pays you a big amount and then charge you tuition afterwards.
From the point of view of a professor, depending on how the school is set up, they don't often think about tuition, so that might be the reason why it was not mentioned. To them, usually the important part is whether or not they will pay for you with their grant (research assistantship) or if you will have to do a lot of TA work to get paid. RA is preferable usually for the student!
The official offer letter should say the tuition waiver specifically though. If you need to know sooner, you can ask the professor about tuition waiver.
TakeruK's post in Are all Ph.D. GA/RA/TAs awarded with tuition waiver? was marked as the answer