If the resistor is 100K , I put my DVM to measure it , it's should read 100K
Only if it's not in a circuit.
In a circuit, other elements may affect the resistance the probes "see".
Imagine that the resistor is in parallel with another 100k resistor. What resistance would you expect to read?
What you are saying is that if there is short it's not going to read 100k it will read 1ohm?
It could read anything between zero ohms and 100k.
What I don't get is that If the 100K is not the short, but there is a short in the circuit somewhere else, will the 100K read 100K or measure as a SHORT?
As I said above, anything from zero to 100k. Your knowledge of the circuit will inform you as to what you should expect, and if you see something that varies greatly from this then you use your analytical skills again to try to determine what may have caused this.
To find shorts in a circuit, do you have to have the power turn on or Off ? can't you find shorts with the power off?
Often off. A short may cause bad things to happen. If a fuse blows or components smoke, or the like, you simply MUST operate with the power off (or removed from that part of the circuit). Must is a strong word -- there are other alternatives (resistors in place of fuses, dim bulbs etc., but you have to know when to use them.
With the power off you will typically use the ohms range. With the power on you will use either the voltage ranges to measure voltage drops, or break the circuit and use the current ranges to directly measure current. Frequently the former is more convenient.
The Tech at my work showed me kinda of something to find shorts, If there is a short in a circuit somewhere, You use the ohm meter can what ever component has a resistance that measures a SHORT on the meter then you have located your shorted network??
I don't really understand what you're saying.
If you mean that once you find the component that is shorted that you have found the short, then that is intuitively obvious.
Cause he was using the Ohm meter measuring the resistance component to component either if its a capacitor or transistor or resistor still using the ohm meter to measure the "resistance of the component" from stage to stage and then he found a stage that made he meter SHORT, so he isolated the network
OK.....
He presumably knew (but maybe did not explain to you) that finding a component with zero resistance across it may not be the final step (the short may be in parallel to the component).
It is possible with a sensitive enough meter to detect differences in resistance caused by the printed tracks and that can also help narrow down the search for the short.