reftex-guess-label-type

reftex-guess-label-type is a variable defined in `reftex-vars.el'.
Its value is

t

• This variable is safe as a file local variable if its value
  satisfies the predicate `(lambda (x) (memq x (quote (nil t))))'.

Documentation:
Non-nil means, `reftex-reference' will try to guess the label type.
To do that, RefTeX will look at the word before the cursor and compare it with
the words given in `reftex-label-alist'. When it finds a match, RefTeX will
immediately offer the correct label menu - otherwise it will prompt you for
a label type. If you set this variable to nil, RefTeX will always prompt.

You can customize this variable.