Many online typing tests do not identify the skill level for each type of test, but instead are based on the number of minutes for the test. The longer the test, the more difficult it can be. The text of some tests may also be more difficult for some people due to the source material.
A helpful way to approach different difficulty levels of typing tests is to use typing test games. These provide typing practice to increase the typing speed, while also testing how fast someone can type during the games. The games at TypingTest.com do actually have difficulty levels to choose from before starting the game, including easy, medium and hard. The medium and hard levels tend to have the words or phrases go by faster, thus making the level more difficult.
Games at FreeTypingGame.net do not have difficulty levels from which to choose, but each game varies in how hard it may be for some players. Some typing test games on this site are "Trick or Type," "Desert Typing Racer" and "Meteor Typing Blast." Some of these games only require a letter or a very easy word to type in order to continue through the game, while others have longer words or phrases.