hello,
???can anyone tell me how to find the COG of quadcopter frame (without electronics)

draw a line between the motors (diagonally) the point at which they intersect is COG.

ok sir

cog is not an essential part in multicopters if your arms (use pine or aluminium hardware) have similar make or length and your centre plate is sandwiched fine, it will come out to be fine, don't worry
you can use ply for this, and cut a square if you don't have access to a laser printer

because you cant always balance a quad in a pinhead at center

actually i am going to use naza lite. and i think exact cog is needed to place MC and GPS antenna

Though it is recommended to place a flight controller accelerometer an exact COG, failing to do so won't affect your flying.

can i place Flight controller on the centre of quad or  i hav to find the exact cog to place FC and GPS antenna

No need to place at EXACT COG - you just have to place the flight controller some what at the middle of the quad

And for GPS and compass, place those away from the motors magnetic fields and if possible at a height above the props .

OK sooraj sir thanx..

jus' 'sooraj' will be enough

ok bro