  The Laser Interferometer Space Antenna, LISA, will detect gravitational wave
signals from Extreme Mass Ratio Inspirals, where a stellar mass compact object
orbits a supermassive black hole and eventually plunges into it. Here we report
on LISA's capability to detect whether the smaller compact object in an Extreme
Mass Ratio Inspiral is endowed with a scalar field, and to measure its scalar
charge -- a dimensionless quantity that acts as a measure of how much scalar
field the object carries. By direct comparison of signals, we show that LISA
will be able to detect and measure the scalar charge with an accuracy of the
order of percent, which is an unprecedented level of precision. This result is
independent of the origin of the scalar field and of the structure and other
properties of the small compact object, so it can be seen as a generic
assessment of LISA's capabilities to detect new fundamental fields.
