How do I test my main board in my 42 LG LED TV?

Lightning strikes kill all the boards and components, 125.000 volts will do that!! Claim on your household insurance or take it to a repairer who has all the test equipment..

Going to need more information to help much, what do the various power board rails look like?...are they all good?

Going by the fact this set has had a lot of main board failures I'd lean towards that board but its just a guess without knowing some voltage measurements

Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis. http://electronicshelponline.blogspot.com/ To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.

Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis.
http://electronicshelponline.blogspot.com/
To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.

Back - light problem. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis.
http://electro-medical.blogspot.com/
To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.

Back - light problem. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis.
http://electronicshelponline.blogspot.com/
Some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement.

It could be the main or power board, either would cause your issue

Check and replace its main digital board. after replacement, somde adjustments have to be done, and you have to enter its service mode option to do it. Really it is a skilled work, that a customer who has no idea in electronics can do.

There are several known issues that can cause that.

The top candidate is the main board, the next is the Y-Main board.

Here's a link for repairing the power supply board of LG 42PG20, which also contains a link to how to test your own board to see if it is good or not:

http://www.coppelltvrepair.com/p/716/lg-42pg20-ua-power-supply-board-repair-service

Hope this helps!

Try the following steps:
1. Remove the power supply to X Main board, check VE. If you get normal VE then replace the X Main board. Otherwise to 2.
2. Remove the power supply to Y Main board, check VE. If you get VE then replace the Y Main board, otherwise to 3.
3. Replace the SMPS board & SMPS 200V board.

Good Luck