how to check if it is R1 or R2, there's nothing printed on
On my board it is printed (above Italy map):
Board Model
UNO R2
Other difference (sorry for english I am French) on the R2 8u2 IC is not parrallel to the AT328 IC but turn 90 degrees.
Other difference (on my board) the IC 5V regulator is in sot223 package instead of Dpack (thermal resistor jonction/case of 15d°/W instead of 6d°/W)-> this decrease the capability of power sharing with shields