Dell Inspiron 1525 laptop won't boot

I've got a Dell Inspiron 1525 laptop that won't boot.

Here's what I know:

• The LCD and memory modules work fine (I've tried both in a different laptop).
• The battery is broken so laptop has to be connected with the charger
• When I press the power button the following happens:
  -- DVD drive makes that power-on noise
  -- Hard drive spins constantly (at least it makes a noise), but it doesn't make that "activity sound"
  -- LCD doesn't power on.
  -- Then it just stays like that.

Here's what I've tried:

• Replaced the memory modules and tried them in different configurations. Ran memtest on them in another laptop with no errors
• Replaced the hard drive.
• Removed the battery.
• Tried all the different keyboard combinations google recommended.

I suspect the motherboard is dead. Is there any way I can be sure that's the case and not something else before I order a replacement?