A QGIS Python snippet to select multipart features in the active layer

find_multipart.py

# Find all multipart features in the active layer
layer = iface.activeLayer()

# Clear the selection first
layer.removeSelection()

# Iterate through features and select the multipart ones
for feature in layer.getFeatures():
    geom = feature.geometry()
    if geom.isMultipart():
        layer.selectByIds([feature.id()])

print('Multipart features selected:', layer.selectedFeatureCount())