My friend recently bought a ruby gem from a store and asked me how to test whether its an original one or a duplicate.
Are there any ways to test it using only domestic materials?
To test if it's ruby at home check it hardness. If it's harder than 7 even if it's not ruby it still something precious. :-) You can also try to find refraction index however for this you need some equipment. If you really want 100% prove, ask on a local chemistry faculty if they could "as a teaching example" make single crystal diffraction experiment. Simple run with 30 frames would be enough to find a cell what is enough to prove it's ruby.
Fitting your ruby on diffractometer would be challenge as a new diffractometers are working on crystals with size of 10--300 micrometers. To find if it natural or synthetic you need to go to specialize jeweler. They look for imperfections (natural stone are usually less perfect than synthetic).