The most practical way is to periodically (say once in 3 months) measure the BP by the electronic machine and follow it up with taking reading by the traditional time tested mercury sphygmomanometer in quick succession. Taking 5 readings sequentially by both the instruments and then averaging the values of readings taken by the electronic machine and the mercury instrument is a good method.
If the average of the readings for both the systolic and diastolic pressure are within 5 mms of each other (as taken by the different instruments), the electronically taken readings are dependable. I do not think it is easy to re-calibrate the readings of an electronic machine. If it has a problem (which is not uncommon), the best thing is to contact the supplier. The gold standard for measuring blood pressure is the mercury manometer.