How does Splinter know who Shredder is? I was thinking about this and it occurred to me that Splinter and the 4 turtles have been in the sewer for all their lives and before then were in the science lab until the fire, so how would Splinter know him if they never ran into one another?

Splinter knows that Sacks is working with Shredder, how? How would he know that? Nothing is given to make us think/believe how Splinter would know this. And sacks back-story as well. Explain please.