Everyone's thought process is different, but here are some things to ask yourself when it's not obvious whether an item should be decluttered or not:
1. Do I like this item?
If you don't actually like it, why are you hanging onto it?
2. Have I used this item in the past year?
If you haven't, chances are you won't. This doesn't really apply to sentimental items like keepsakes from your childhood, but for items like clothes and small appliances, it's a good indication of whether you're getting value out of the item.
3. If something happened to this item, would I replace it?
Are you holding onto this item out of guilt or because you think you will use it one day? If it disappeared or broke, would you run out and replace it, or do you secretly hope it will just vanish?
4. Does this item make me happy?
Many of us hold onto things that make us a little sad. Logic says that if it's a perfectly good object, it should be kept. I disagree. I've kept perfectly useful things that remind me of my ex. What exactly was the point of keeping those things for so long? Give them a new home.
5. Could someone else get more use out of this item than I do?
In the case of items that are only used occasionally, ask yourself whether it's worth taking up valuable space in your home.
6. Does this item prevent me from doing things?
Is your basement or spare bedroom so full of crap that you can't use the space? Is your closet so full that you lose track of what clothes you own? Is cleaning becoming impossible because of all the knick knacks everywhere?
7. Could this space be better used?
This is similar to point number 6. Most of us have a finite amount of space to keep our stuff. Use that space wisely.
8. Is this item worth having around?
Most stuff we own has some sort of cost associated with it. Consider whether owning this item costs you money, time, or energy, and declutter accordingly.
You can read more of my articles on decluttering here.
How do you decide what to declutter?