is there a way to call the existing value of cell inside a UDF?

Cell A1 has a formula in it, including a UDF.
Inside that UDF's code, is there a way to retreive what the existing value
of A1 is, before the UDF replaces that value with a new one.

intended purpose:
to prevent a volitile function from running every time Excel refreashes its
calculations

nonexistant code example:
IF(ISNUMBER(CellValueFunction()),cell_value_functi on,rand())

Thanks