The closest function in OpenFHE 1.5.1 is SerializeEvalAutomorphismKey. You can serialize the keys and look at the size of the serialized file (all rotation keys are stored together). Make sure that you only use EvalBootstrapKeyGen in this experiment, i.e., no other rotation keys are created via EvalRotateKeyGen, EvalAtIndex, EvalSumKeyGen, etc. You can also serialize the EvalMult key, by running SerializeEvalMultKey - this will give you the size of one evaluation key. You can divide the size produced by SerializeEvalAutomorphismKey by the size of EvalMult key to get the total number of rotation keys.
In v1.6, we will expose a function that returns a vector (python list) of indices, i.e., we will provide a direct method.