Sebastian already pointed out the most stable - if awkward - way, using the stateflow api search methods, though you have to be careful to search the library if a stateflow block resides there and is only linked into your system.
There is another way only using the simulink api, as you required. Using find_system(gcb ,'MaskType', 'Stateflow') still finds the block, even though get_param will yield an empty MaskType on the very same. Seems to be some kind of backwards compatibility programmed into find_system, that is still working up to 2016b. Also awkward, but pure Simulink-API.
As Embedded Matlab blocks are technically integrated as Stateflow blocks into Simulink, these will be matched by find_system(... 'Stateflow') as well. So if you need to distinguish the different kinds of Stateflow embedments into Simulink, there is no way around Stateflow API.