People who use Service Animals are independent and want to be treated that way.
They will certainly ask if they require assistance; if you feel they may need assistance, simply say, "May I help you?" The Service Animal will usually be on the left side of a person who is blind, so if the person needs assistance, approach from the right, or opposite side of the Service Animal and offer your arm.
If the person who is blind is seeking assistance to cross the street, take them all the way across and up onto the opposite curb where the dog will know to resume his duties. Do not pet a Service Animal when it is in harness. Always ask the handlers permission as to whether you can pet the dog or not.
Never feed a Service Animal. The handler looks after the dog's diet very carefully, and the dog works most efficiently when the recommended diet is followed.
If a Service Animal is wearing its harness, vest, or backpacks, this means it is working and should never be patted or distracted by having its name called, be given excessive eye contact, noises made at it, or made the center of attention. These may all cause the dog to take its concentration off the job and put the handler’s safety at risk.
(info from dogguides.com)