[Fabman]

Can I check then through the on/off method or do I have to get a reader?

I have a 2001 Durango 4.7 4x4

 

[Sneezer]

I believe you can. You can also hop on over to your local Autozone. They will check them for free.

 

[DirtyR/T]

00-04 model you can use the on/off trick

 

[GOON'S D]

on/off 3 times will do it for you!