It is generally done by comparing the integrated fluorescence emission spectrum of the compound of interest to that of another compound with a known quantum yield, as described in the documents below. Try to choose a standard with similar fluorescence properties to your substance, and use the same solvent for both. The quantum yields of commercial dyes can often be found in the vendor's catalog (see for example the Molecular Probes Handbook). They are solvent-dependent.

http://www.fluortools.com/software/ae/documentation/qy

https://www.perkinelmer.com/lab-solutions/resources/docs/APP_Determination_of_Relative_FluorescenceQuantum_Yields_using_FL6500_Fluorescence_Spect.pdf

https://www.thermofisher.com/us/en/home/references/molecular-probes-the-handbook.html

Thankyou Adam B Shapiro Sir.