Acronicta lepusculina - Cottonwood Dagger Moth, images in the guide here? Nice find!

Is this a definite ? If so, why is this one white and the ones in the guide yellow ?
Also, this may be a stupid question but how can you tell if it's a beetle larvae, moth, or butterfly ? Is it just experience or is there something specific to look for ?

Many species have a range of possible colors. This one white to greenish white to yellow. Others, such as lady beetles or spittle bugs are painfully variable!
Your second question is tougher, but you're right that experience goes a long way. But just some clues: Fly larvae tend not to have any visible legs. Beetle larvae will have six legs up front on the thorax. Butterfly and moth larvae will have six legs on the thorax and additional pairs of prolegs supporting the abdomen, giving them that classic caterpillar walk. Usually they have 4 pair on the back abdomen segments and one pair of anal prolegs at the very back. In some species these are reduced till you reach the Geometrids which usually only have one pair of abdominal prolegs and one pair of anal prolegs giving them the classic inchworm walk. Sawfly larvae usually have even more pairs of abdominal prolegs, 5 pair or six, which is usually quite noticeable in the photos. Beyond this there are many differences in the head and mouthparts between these groups. Scroll through the guide examining larvae and you will begin to get the gestalt of the different groups. Good luck and enjoy your learning!

Great info and I really appreciate it!
And, I really am having fun learning. Someday I'll feel more comfortable about ID's when I learn more of what to look for. Just so you know though, I have been trying to ID some of mine based on the refs I have and the refs in the guide. But, there's just so much to know and look for so a lot of my calls have been wrong and that's ok because that's how you learn.

I'll have to remember this one, as I've never encountered one in the wild.