1872 us trade dollar 420 grains 900 fine how to tell if it is counterfeit?

It is a fake without a doubt. Trade dollars were not struck until 1873, any trade dollar bearing the date of 1872 is a counterfeit.

There are numerous iron (magnetic) fakes made of the 1872 dollar, which copied the genuine "pattern coins" struck in 1873 but dated 1872, of which 6 are known.