Hard because there are so many that claim to be Christian. Easy because when you have a principle to follow there isn't a problem.
Here is/are the principle(s): Before committing to any group, make sure they believe the Bible, are willing to change when they see their errors, and are not so fast in making decisions that they forget what is important.
With a foundation in the teachings of the Bible, it won't matter what denomination. All of the core principles of Christianity come from the Bible, so you can be sure you'll be okay in a place where they look to God's Word.
If in doubt, you can search for the denomination name on the cultwatch website to see if they are dodgy. Another resource of Cults and Religious groups is here.