To determine if a moss gene found in Amborella is functional, there are several possible avenues to explore. One would be to check if the gene has all of the features of a functional gene (promoter, start and stop codons, no oddly placed stop codons, etc.), and another would be to check a database to see if the gene is homologous with one of known function. A third way to check would be to use a mutant plant lacking the putative function of the gene and add back a non-mutant copy of the gene to see if the function were restored to the mutant plant.
Work Step by Step
To get to this answer, review previous chapters on genetics for the things that determine if a gene is functional and what makes it so: its sequence, its basic structure, the shape of the protein which it codes for. Also review how you use information like sequence databases to analyze this. Ch. 24 is a good place for much of this review.