rental yield (Condo) can be found on URA. they provide a 3months update each time. However it can only be used as a guide (only those legally stamp duty rental are registered). Location, Demand and alot of other factors play a part in the rental rates.
Rental yield for landed, you could use the psf as a guide. Do a search of the properties you wish to compare, do a detailed compariative market analysis (Current rental, Rental around the same area, compare apple to apple). Well if rental for landed can be easily compared, URA would have a report on it (they have to answer to the government on any trends).