By the context of the sentence. It should be obvious.

I just want to add, there is no difference in the pronunciation... they sound the same.

[Stationery]: pens, pencils, rulers,..etc and it is a noun [Stationary] : stable, Not moving. Not capable of being moved; fixed it is an adjective. so, it should be followed by a Subject e.g: (In the Park there are stationary Benches. It can't be moved from its place) ========= So, as Mr. Eliot said, it will be obvious by the context of the sentence.. -- I hope I helped (^_^; )