General inspection does not refer to some inspector walking around, but simply means for all to see.
General here has the same meaning as in general public, and inspection just means the act of looking at something.
According to some sources, this is a reference to Shakespeare's Othello:
But I will wear my heart upon my sleeve
For daws to peck at: I am not what I am.
In this pdf we find the following explanation for the daws and the meaning of wearing one's heart "outside":
Daws, aka Jackdaws, are a kind of bird, related to crows.
The line in Othello, and here [in A Christmas Carol], refers to honesty and openness - the wearing of one’s true character in the open for everyone to see.
Basically, Charles Dickens is describing that the grocer and his people were very pure of heart. They used shiny heart-shaped fastenings on their apron, and Dickens sees in those polished hearts a symbol of them showing very honestly who they are: very frank and fresh people.