Okay, my mother-in-laws vehicle died on her about 45 min. from home, so bear with me on the vaguness....
If possible I would love a "checklist" of items to go through to hopefully discover the cause of the engine not starting. Recently a similar episode happened and thankfully a new battery appeared to solve the problem until this. She said the car wouldn't start up when a friend went to leave work. Nothing was accidentally lefton, and as I said the battery was recently replaced. She said the engine cranks over just fine, but it just wouldn't start. Almost as if it wasn't getting gas. (A little background... after the battery replacement, I did a once-over under the hood to visually inspect the vehicle because I had my doubts about the person who replaced the battery. I noticed the timing belt was cracked and was going to need to be replaced, and pointed it out to this person. As far as I know, it has not broke since they should have mentioned this to her since I had already pointed it out! Also, she recently had the local teenagers running the qwickie-lube replace her fuel filter because I was taking it to be smogged on the 1st of this month.)
With that all being said, can anyone give me a quick run down of likely causes/ things to check so I can help her avoid paying someone to go to the relatives house it's now at to do the repairs. Any help would be much appreciated!

How could you see the timing belt without removing the cover?

Not talking about the serp belt are you?

Anytime I hear of a no-start DOHC 3.4 I cringe...usually the timing belt broke. If it broke while running there may be damage inside, even though it's supposed to be a non-interference motor...sometimes the valves get tapped anyway...

Tell them to take the oil cap off and have someone crank it, look down in there and see if the valves/cams are moving. If not, belt time...

Otherwise it could be many things...crank/cam position sensor, ignition module, fuel pump, etc...just have to do the standard checks for spark and fuel to get started down the right track.