May be you could have an argument to sub B:

sub B(byval sCallerProc as string)
if sCallerProc=A then
....
else
....
endif
end sub

that won't work if B is called directly by the user. It will need to fire
sub C that will fire sub B.

I'm not sure if I'm clear.

I didn't follow the Sub C bit, I'm afraid. Gary's reply covers it, with the
optional argument.