Biodiesel is a biofuel that is of great importance; its good performance is associated with its purity mainly related to a high content of fatty acid alkyl esters, quantified by gas chromatography (GC), liquid chromatography (LC), hydrogen nuclear magnetic resonance (1H-NMR) and infrared (FTIR) spectroscopy. However, these methodologies take long a time and are relatively expensive. Herein, thermogravimetric analysis (TGA) and viscometry (VA) are studied as alternative methods for the quantification of alkyl esters (FAME and FAEE). Analytical curves from the viscosity and mass loss percentage (TGA) as a function of methyl and ethyl ester levels on binary mixtures (soybean biodiesel/soybean oil) were constructed. The correlations experimentally obtained were presented and used to quantify biodiesel content in the products of ethanolysis and methanolysis and in biodiesel/vegetable oil mixtures of different feedstocks. This comparative study confirmed that both alternative methods are adequate to be used. A single viscometry analytical curve could be used to analyze products, obtained by different oleaginous plants, if their oils exhibit similar viscosities. Viscometry can be the most suitable alternative technique because it is portable, fast, less sensitive to the presence of intermediates than thermogravimetric analysis, easy to work with and very inexpensive.