how to calibrate temperature sensor IC DS1621?
In general the best way to find out if you need calibration is to check manufacturer's website. There you will probably be able to find product datasheet which will probably contain needed information. 

If the datasheet isn't available on manufacturer's website (or there isn't any website), search for model_number datasheet. There are several sites on the Internet which catalog datasheets, so you may be able to obtain it even manufacturer isn't providing it anymore.

For your specific case, here is the product site. Under technical documents, you will find datasheet.

On the second page, under OPERATION it is said that the sensor is already calibrated for measuring temperature in Celsius degrees, so no further calibration should be needed.

Furthermore is you read the command set part of the datasheet, you will find out that there is no command for calibration, so it should be impossible for user to calibrate the sensor.

If you are having accuracy problems, take a look at page 13 of the datasheet. Accuracy is related to voltage, as show in the DC electrical characteristics table.