Is it always possible to determine the genotype by phenotype?

No. The dominant gene suppresses the effects of the recessive. As a result, a trait that is encoded by the dominant gene appears in the phenotype. In heterozygous organisms, there is both a dominant and a recessive gene, however, only one (dominant) appears in the phenotype.
According to the phenotype, it is not always possible to understand which genes a given individual contains. For example, in a pea plant with yellow seeds, the genotype can be either AA or Aa. But the recessive trait appears only in homozygous plants with the aa genotype. In other words, we always know what the genotype of an individual with a recessive trait is.
According to the phenotype, it is not always possible to understand, because phenotypic traits defined by particular genes under different conditions of existence develop differently.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.