"The SDCC docs don't indicate to me what the C syntax is for this."

Of course they don't: the purpose of the Compiler manual is to tell you about the specifics of the Compiler - not to teach you the 'C' language!

You will need to read a 'C' textbook (and preferably attend a 'C' course) to find out about standard 'C' syntax.

The standard, portable 'C' way to test individual bits is by masking.
As already mentioned, the 8051 has certain bit-addressing facilities. This is where you do need to consult the Manual - to see if the compiler provides any special, non-standard language extensions to allow you to take advantage of this in 'C'.
Keil certainly does.