When the guy with the suit on comes in to Over Our Heads, Tootie tells Blair that something is off about this guy and she thinks he is a shoplifter. Blair turns to Jo and says: "is he a shoplifter Jo?" Jo responds: "How would I know?"
Jo should know because she shoplifted in the episode "shoplifting" for Mrs. Garrett's birthday present. Its just a little humor.

..or it could be a comment on Jo's keen street sense and ability to read people.

I think what Jo meant by saying how should I now is cause she was tired of them always assuming she would know those things.

The main things that highlighted the episode was the brand new "Over Our Heads", which shifted the show from a gourmet country-esque store to a contemporary store of the 1980's. And Jo no longer had that Bronx barbarian attitude.

Blair turns to Jo and says: "is he a shoplifter Jo?" Jo responds: "How would I know?"
Jo should know because she shoplifted in the episode "shoplifting" for Mrs. Garrett's birthday present. Its just a little humor.

What about the season 5 episode that introduces that obnoxious "street smart girl" where Jo throws her out within 5 minutes for shoplifting, taking things out of her jacket?

Because Jo caught her.