Just remove the chain and move the derailleur by hand back and forth to see the spacing between the cogs and the guide pulley (the one closest to the cogs). Adjust by turning the B-screw. You normally want that around 5-10mm from the cogs to the pulley at its closest point depending on manufacturer (I set based on the big cog with an 11-25/28 and SRAM). It can be checked and set with the chain on but it's a lot easier without.