Talk to the alien for a while. If he/she, he-she or it can not use contractions, you know you got an alien on your hands. Either the outerspace type or just the foreigner variety. This occurred to me as I was watching a re-run of Kung-Fu tonight. Neither Kwai Chang Caine nor some Indian kid that he meets can speak English with contractions. Although I guess technically the Indian kid is not an alien, compared to the White Man.
I cannot use contractions but I will not hesitate to kung fu your ass if you do not learn my name is Caine, not "Chinaman."