Power Tests
Several traits and abilities request that a character performs a Power Test. This is a randomised test with a 25% chance of failure.
Unless otherwise specified, you can always delay a test until it would not interrupt the flow of roleplay or combat. Some powers require you to make a Power Test at a specific point in your roleplay, notably Medicae and Exorcisms.
The standard suggestion for doing this is to take a bag of beads and fill it with 4 beads, 3 of one colour (representing success), 1 of another (representing failure) and do a draw. However, other randomisation methods capable of determining a 25% chance of failure are acceptable, for example random number generators, spinners, or dice rollers but check with the Game Team first to ensure there are no issues (asking other players to "pick a number between 1 and 4" for example is not appropriate).
There may be conditions where the odds will flip, and a test will be performed with a 75% chance of failure and a 25% chance of success so whatever method you choose ensure that it can be inverted in some fashion.
Random Effects Decks
Some skills require a draw from a randomised effects deck, often after failing a Power Test (though sometimes by default - for example, when a character is reduced to 0 Endurance by the BOLT, BURN or SUNDER calls).
PDF files for these decks can be found on the Rules Downloads page. These decks are required for characters with the Exorcist, Medicae, Chirurgeon and Psyker traits. Players should print out copies of the decks to carry on their person and blind-draw a card when prompted by the rules.