Most power transistors, power diodes, power FETs' I have seen are ground
How do you know if the tab or flange on the device is not ground? any test I can do with my DVM meter or the datasheets will tell?
Any reason why a designer would choose components or devices that was not grounded or needed positive ground?
I have never seen a transistor or diode that had a ground connection as it came out of the box. How a component is connected is purely a matter of what the design engineer wants it to do in his circuit.
As for how an engineer uses those components tend to be dictated by how components that he needs are commonly built by the manufacturers.
Mosfets and BJTs that come in TO-220 and similar packages usually have their drain terminal connected to the tab. (See crutschows post #6) For an N polarity device, that means the positive part of the circuit needs to be connected to the tab. If the tab is directly connected to the heatsink, the heatsink will be positive.
The next power component likely to be on a heatsink will be the rectifier. Again, most rectifiers in tabbed packages have the cathode on the tab. Since the cathode is going to be the positive output of the rectifier, the heatsink, if not insulated from the tab, will be positive.
But one thing that you need to be aware of, is that it really doesn't matter which polarity is connected to the heatsink in a high voltage circuit. A shock hazard is present anytime you have high voltage in a circuit. Even if there are no heatsinks. This is especially true if it is the front end of an off-line switching power supply. Even if the heatsinks were at 'ground' (the negative terminal of the bulk capacitor) it would still be at a lethal voltage with respect to earth ground.
Billy, some of the questions you have asked have been of the "why is a product built this way", or "why component X was chosen". There are no universal answers to those questions. If you give the same product spec to 10 different engineers, you will end up with 10 different designs. Granted, 8 or 9 of them may be similar as there are 'common' ways to approach many tasks. But they won't all be the same down to each individual component.
Also, engineering is always about trade-offs. Product safety, cost, size, efficiency, component count, ease of assembly, components already in use in other products, etc. All of these affect each other in sometimes very complex ways. An engineer will take lots of factors into account when working through a design task.