From the emacs manual's section on minor modes:
Most minor modes also have a "mode variable", with the same name as
the mode command. Its value is non-
nil if the mode is enabled, and
nil if it is disabled.
For example, associated to
abbrev-mode, the function and minor mode, there is the variable
abbrev-mode whose documentation is:
abbrev-mode is a variable defined in `C source code'. Its value is nil
Automatically becomes buffer-local when set. This variable is safe
as a file local variable if its value satisfies the predicate
Documentation: Non-nil if Abbrev mode is enabled. Use the command
abbrev-mode to change this variable.
As this indicates, in general the variable should be changed via the command.
As mentioned in the comments the case of
indent-tab-mode is unusual, there is no corresponding function, just the variable. Anyway you can test for whether it is set or not via