WordNet sense 1 (show approval or appreciation of):

recognise

WordNet sense 2 (grant credentials to):

accredit

WordNet sense 3 (detect with the senses):

discern, distinguish, make out, pick out, tell apart

WordNet sense 4 (express greetings upon meeting someone):

greet

WordNet sense 5 (express obligation, thanks, or gratitude for):

acknowledge

WordNet sense 6 (be fully aware or cognizant of):

realize, agnise, agnize, realise

WordNet sense 7 (accept (someone) to be what is claimed or accept his power and authority):

know