How can i confirm that the given pedigree chart is of an autosomal dominant trait?
Sure, both are possible here!

Your book probably makes an implicit assumption of low frequency for the "red allele".

Assuming, "red" means "disease", it is quite standard to assume that the "red allele" is at low frequency in the population. If this is the case, then it is unlikely that partners of the members of the main family being represented in the tree carry this allele. It is therefore quite standard to assume that partners does not carry the "red allele". If this assumption is made, then recessive autosomal cannot explain the observed tree.