Usually, it’s glorot_uniform by default. Different layer types might have different default kernel_initializer. When in doubt, just look in the source code. For example, for Dense layer:

class Dense(Layer):
...
    def __init__(self, units,
                 activation=None,
                 use_bias=True,
                 kernel_initializer="glorot_uniform",
                 bias_initializer="zeros",
                 kernel_regularizer=None,
                 bias_regularizer=None,
                 activity_regularizer=None,
                 kernel_constraint=None,
                 bias_constraint=None,
                 **kwargs):