Question

how we can identify that transformer is step up or step down ??

Answer 1

Often it is obvious, from the design of the terminals.  A doorbell transformer, with its screw terminal outputs,  is a common example of this.  Otherwise, a measurement of the dc resistance of the windings will give you an idea.  If the transformer is a 10:1 step down transformer (Example, 120Vac input, 12Vac output) then the dc resistance of the primary (120Vac winding) will be perhaps 100 times greater than that of the secondary.  Both dc resistances may be quite low, however, so you may need a good ohmmeter to make the measurement.