Elle, Thanks for the reply and appreciate your response for the
following:

1.If the new radiator was not put back correctly by the mechanic, can
the coolant leak into the oil system?

2.If the coolant leaked into the oil system, can I expect to find a
milky white oil when I check the inside of the oil cap?
I will also check the coolant level and oil in the coolant system as
you have mentioned.

3.From all the helpful reply, I strongly suspect the coolant has
leaked. The reason for my suspicion is - When I start the engine when
it is dead cold, there is no smoke but there is small shudder when I
start the car. There is no smoke all the way to college which is 10
miles (it takes 20 minutes drive this 10 miles). When I start the
engine 15 minutes after the engine is hot of driving 10 miles, I see
white smoke and the car shudders a lot. When I reach the college and
instead of starting after 15 minutes but start after 11 hours (the
engine is dead cold), there is no smoke. So I see smoke only when the
engine is hot and you had mentioned that it take high temperature for
coolant to convert to steam. Is my deduction some what correct that
coolant has leaked into the oil system?