See the "-a" option to cksum(1).
Type "apropos sha256" at the prompt, pick something that
looks promising (e.g., SHA256_Init) and open its man page.
Don't get dejected if the first thing you try isn't what
you might have hoped to see. Take a peek at the "SEE ALSO"
section of the man page for something that looks promising.
Continue as necessary.
The first works in this case, but the second (supported by
the FAQ, the various mailing list archives and a bit of
patience) seems to work pretty much all the time (mutatis
mutandis). At least it does on OpenBSD.