sound out

PHRASAL VERB

If you sound someone out, you question them in order to find out what their opinion is about something.

• He is sounding out Middle Eastern governments on ways to resolve the conflict.
• Sound him out gradually. Make sure it is what he really wants.

... Cobuild

sound somebody out (also sound out somebody):

to carefully discover what someone thinks or knows

• I thought it might be good to sound him out about having you come to work for us.
• His policy was to sound out top business leaders before making any new economic proposals.
  
  Usage notes: used to describe a way of asking about someone's opinions without upsetting or angering them

... Cambridge Dictionary of American Idioms,

http://idioms.thefreedictionary.com/sound+out

• Trades carefully studied his forecasts, and salesppeople sounded him out before talking to clients.