Some of the puzzles posted here are hard. Others are difficult. There's really no way to easily tell what a puzzle's difficulty level will be - and sometimes the writer of the puzzle will misjudge the level of difficulty as well.
You can sometimes tell by who posted it - generally, puzzles that I post will be on the easier side, while puzzles by Alconja will often be amazingly difficult.
If you want to try your hand at something, you can always look at whatever new puzzles pop up and take a look at them. Some will be hard. Some will be easy. Some will be awful, and some will be astounding. But there's really no easy way to tell what will be what, for easy finding.
If you want to find specifically mathematics and logical-deduction puzzles, you can follow a tag (see How do I follow a tag? on Meta.SE), as well as add the tags as favorites on the right sidebar on the main page to make these puzzles appear with a highlight on question pages.