The high, insect-like reeling song of the Grasshopper Warbler is the best clue to its presence.
The sound of the Grasshopper Warbler is significantly higher pitched than the sound of the Savi's Warbler (in Dutch: Snor) and the individual pulses (reeling sound) are almost twice as slow.
The Grasshopper Warbler's song sounds less "buzzy" than the sound of  the Savi's Warbler.

1st photo: Naardermeer, Naarden, The Netherlands, 26 April 2014
2nd & 3rd photo: Biesbosch, Sliedrecht, The Netherlands, 6 July 2013
4th photo: Bird banding site, Naarden, The Netherlands, 8 August 2010