Are all unicellular organisms haploid? If not, how do we know a cell is haploid or diploid?
All unicellular organisms are not haploid.
All chromosomes are different in appearance as far as the length and position of centromere is concerned in haploid cells . Haploid number means a set of chromosomes which are different from one another in morphology and the genes present on these.
Complete set of chromosomes is present in duplicate in diploid cell. Each chromosome is present in duplicate. Two chromosomes in a diploid cells are identical to each other in their morphology, i.e. length and the position of centromere. The genes present on two chromosomes on identical loci represent same character but may bear same allele or alternate allele of that gene. These two chromosomes are termed homologous chromosmes.
Thus we can easily identify whether given cell is haploid or diploid. If all the chromosomes are different from one another in morphology, the cell is haploid. If every chromosome has its homologous or duplicate chromosome that can eassily be observed, the cell is diploid.