I have seen that trace impedances in PCB's are maintained 50 ohm & so on. How to measure this impedance using a simple multimeter?
There is no* way to measure trace impedance with a simple multimeter.
Measuring trace impedance requires, at minimum, an oscilloscope and an impulse generator of some sort. Here's an article about how you can measure the impedance of a cable (or PCB) with a signal generator.
Doing "Proper" impedance measurements requires a special tool, called a network analyzer.
* No reasonable way, in any event. There may be ways to abuse a specific model of meter in a specific manner to get a approximate result, but it won't be generalizable.
You should use a TDR (time domain reflectometer) or a Lecroy Sparq which is a cross between a network analyzer and a TDR. You can also use controlled impedance with your board house and specify to them what the impedance is. Then demand a test coupon and measurement results.
As a poor mans approach you can sort of build your own TDR with your oscilloscope. But you need a fast edge / pulse source, probably in the ps range depending on how long your traces are. Preferably with an output impedance close to your trace. I once used a analog devices high speed opamp with ps rise on the output for this.
Anyway input the fast pulse into your trace ( with no termination) and if you measure with your scope at the source you should see the reflection from the unterminated edge come back.
Then add a termination resistor at the far end of the trace that you think matches the impedance. When you get it right no more reflection :)
A little tricky but doable if you can't afford the test gear.