What is the smartest way ?

[Jane]

Laptop ACER aspire 1 a114-32 (n17q4 ) has dark screen. If I push On/Off button the laptop seems to start ( including blue led light) but no picture.
What is the best way how to identify the reason? Whether is a culprit LCD panel or LVDS cable or mainboard ?

[rogfanther]

Connect it to an external display. Does it show image correctly then ?

[FriedFred]

Light-in with a torch. If you can see a picture, then the background-light is off.

[pc_okay]

So Many possibility!
After testing other suggestion you may try:
Checking caps lock too, does it respond when you press it?

[Jane]

Checking caps lock too, does it respond when you press it

Respond, how?

[pc_okay]

Sorry, if there is no caps lock indicator on this laptop plug an USB keyboard with an indicator.
The caps lock LED will light ON and light OFF when you press it if laptop is POST.

If nothing happen it may not be a screen problem.

[Jane]

Thank you for the explanation.