since you are asking this means you have doubt. hence as long as it is not bought new from AD, i would strongly suggest that you get it authenticated...
My 18K Gold Day Date "President" Collection:
1) WG DIA 18239
2) YG DIA 18238
3) WG 18239
4) YG 18238
5) YG 18248G Bark
6) WG DIA 18039
7) WG 18039
8) YG 18038
9) Tridor DIA 18039B
10) YG DIA 18078 Bark
11) RG 1803 (Mint V.Rare)
12) WG 1803
13) YG 1803