(I) can't say for sure
I don't know with certainty. A: "How many people are coming to dinner tonight?" B: "I can't say for sure—my family tends to just show up without responding to invitations."
(I) can't say for sure.
Inf. I do not know with any certainty. Tom: When will the next train come through? Jane: I can't say for sure. Bob: How can the driver hit so many potholes? Bill: Can't say for sure. I know he doesn't see too well, though.