I might have an original signed painting by Moretti - how do I know for sure - it looks like the same style of signature - I bought it at a thrift store.

The only way to know for certain is to take it to an expert who can examine it. Signatures are one aspect they will look at. Brush strokes, paint type and quality, subject matter etc. are all things they will look at to know if it is an original, copy, or fake.

There are no shortcuts in authenticating an object.