Kobolds.

From Judika Illes' "Encyclopedia Of Spirits"
This is summed up it's a lot longer. just getting important parts out.
Kobolds are shape shifting beings typically with the affinity and addiction to people. They usually will find their homes in caves or trees. Some times even blending in and going into cities!

One thing though that's great about them. People will ask them to be their magickal servants and they will always be happy to oblige. If they feel a real bond with the certain person, they will work tirelessly on their behalf. There is one problem though. Only ask one to be your servant if you are only considering a close relationship. They can not sever ties with their master. They hate being away from him/her and will almost never leave them. If they are trained well enough they will leave you alone to do work and maybe even sleep in your room!

How to find a kobold:
1. A tree houusing a kobold must be located
2. Take a small piece of wood from the tree and thank it for it's abundance, it will come to you one way or another.
3.The ideal method is to bring gifts to the tree such as water or honey. Explain the purpose and the kobold will willingly accept you.
4. Branches that fall nearby or from the tree is an affirmitave answer (some are more quiet and tend to just sneak in)
5. Offerings should me made regularly to the kobold to give it energy for it's work. It's just like feeding a pet. They will usually just eat anything you didn't eat off of your plate, or if you love them even more just make them a meal instead.

Remember midsummers eve is the night for rituals intended to locate kobolds.
I'm sorry but it doesn't say how you know whether a tree has a kobold in it or not. you could hear a faint knocking or get an energy that brings you to it.

This sounds cool! I have a tree out in my yard that gives me a weird vibe. I wonder it has one.

could be. good luck!