How do the people who make tv shows know how well the pilot does after its first airing?

The pilot is actually made for the network to decide on the show before they air it.  The way that they've traditionally made tv shows for networks is that they start with a basic pitch for a whole bunch of shows and then they go through several rounds of narrowing it down.  Then there's a final crop which they feel interested in enough to actually invest real money in.  So they pay to make one episode of the show, a pilot, in order to get a better sense of things like how the show will actually look on screen, what the actors will be like, etc.  They then decide whether to air the show after that.  (in a few rare cases, like Buffy and Game of Thrones, they'll actually pay for a second pilot because they're interested enough in the concept but the original pilot just doesn't work).

In terms of how well a show does, this is measured by ratings.  In the US that basically means the Nielsen company, which has been measuring tv viewership pretty much since the medium began in the 50s.  They use a sample of households which are supposed to be representative of the population as a whole.  They track viewership through devices attached to the tv which show what channel its tuned to.  Since the advent of streaming, the streaming companies also have direct info on how much a show is watched.  So if you watch Grey's Anatomy, for example, on Hulu the day after it airs, they can tell that.  The networks can then take this ratings information and use it to determine if they want to keep airing a show.  It's pretty rare for a show to get cancelled after the first few episodes.  It usually has to really bomb to do that.

They screen the pilot to live audiences and get their reactions

I would imagine they look at a graph and if the graph shows a good trend then they continue