Thermal Pad is needed or not?

How to tell whether a thermal pad is needed or not from seeing mosfet datasheet. I want to put an H bridge on a Heat Sink. so for Electrical isolation I need Thermal pads, but some of the switches dont need becasue they have already insulated case and only thermal paste is fine. So how to tell?
For example refer these two Datasheets
Datasheet1

Look at the pinout. If it's connected to something, it has to be the same as a pre-existing electrical connection and thus will be listed alongside all the other pins.

One cannot tell from those datasheets if the case is "isolated", I have my doubts they are. That vendor does have isolated devices, called "ISO247" or "ISOPLUS247" packaging such as IXYJ 20N120C3D1 under "ISOPLUS" Technology.
See:
https://www.ixys.com/Documents/IXYS_Selectorguide_2015.pdf

The isolated MOSFET devices seem to have a part number with an "R" in them, like IXFR16N120P

You should be able to see it by inspection. If not, try an Ohmeter...