1.how would you differentiate your heart rate before and after exercising?
2.what is the advantage or timing for a full minute to find your pulse rate?

Your heart rate before exercising would probably be the at rest rate of say about 50-60 beats per minute whereas immediately after exercise it could easily be say 115-130 and depending on age there is an optimum heart rate for each age range which decreases somewhat with age. Timing for a full minute your heart rate is best as it will give the actual count whereas if you just do it for 10 seconds and then multply by 6  it would assume it will be consistent whereas it could decrease during the minute so the actual measured count for a full minute is more accurate.