Based on @andrei-stefan answer, you can try also:

environment.getPropertySources()
    .stream()
    .filter(MapPropertySource.class::isInstance)
    .map(MapPropertySource.class::cast)
    .map(MapPropertySource::getPropertyNames)
    .flatMap(Arrays::stream)
    //.anyMatch(propertyName -> propertyName.startsWith(key));
    .anyMatch(propertyName -> propertyName.equal(key));