I'd cut them a little slack. I think it's often a question asked just for something to talks about, an ice-breaker. There's no malice in it. When Scooby was brought to our house, I asked the chap who he was with "is a a full Lab?" - partly for conversation and partly because he was taller and narrower than my idea of a Lab.
If I had a dollar for every time somebody asked this question I do feel better now that I know that some of the most Labbiest looking Labs on the forum get asked this question, too. The Animal Control officer who was here the other day was surprised to hear that Xena was 2, she's so small.
Yes I do find people use this as an ice breaker more or less. But I’m asked any time I come across a person. Every. Single. Time. I just smile and say ‘he’s a lab’
That's my usual. "How old is your puppy?" Erm.....she's just turned 8......cue I am really really rubbish at dog breeds if it's not a lab though and always preface any "what a lovely dog" conversation with "I'm rubbish at dog breeds.....what a lovely [pause to allow dog owner to prompt me to the breed/cross]" technique.
For our first year or more of having Homer Hubby would point out any reddish brown dog and say it was a lab like Homer and any small dog below knee hight was a puppy.
We get that asked if Chewie is really a lab because he has a white marking on his chest. If we really want to bore people we start talking about the St John's water dog!