Intrinsically disordered linkers control avidity, auto-inhibition, catalysis, and liquid-liquid phase separation in multidomain proteins. Linkers enforce effective concentrations that directly affect the kinetics and equilibrium positions of intramolecular reactions. Mechanistic understanding of the role of linkers thus requires measurements of the effective concentrations in supramolecular complexes. Here, we describe an experimental protocol for measuring the effective concentrations enforced by a linker using a competition assay. The experiment uses a FRET biosensor that is titrated by a competitor peptide. The assay is designed for parallel analysis of several constructs in a fluorescent plate reader and has been used to study hundreds of synthetic disordered linkers.