Here's how they know:

* The Human Genome Project: This monumental project, completed in 2003, mapped the entire human genome. This means scientists identified the location of all the genes on each of our 23 chromosomes.

* Genetic Sequencing: Scientists can sequence the DNA of an individual and compare it to the known human genome. This allows them to pinpoint the exact location of specific genes.

* Genetic Mapping: Techniques like linkage analysis and pedigree studies help researchers understand the relationship between genes and their location on chromosomes.

However, it's important to note:

* Ongoing research: While we have a good understanding of gene locations, research is ongoing to discover new genes and refine our understanding of their locations.

* Variations: There can be slight variations in gene locations between individuals due to genetic mutations.

* Complexities: Genes are not always neatly arranged on chromosomes. Sometimes they overlap, or their functions can be influenced by other genes located nearby.

Overall, scientists have a detailed understanding of gene locations, but the field is constantly evolving and becoming more sophisticated.