Czechs don't usually look like this, having often broader, more Western-European, i.e. "Germanic" faces:-)
Genetically speaking, Czechs are a mixture of Slavs on one hand (about 51%) and Celto-German blood (major part of the rest) on the other or so does the Czech press say. These findings seem to be confirmed in the history: what is now Czechia was first inhabited by the Celts (hence its maedival name: Bohemia), then conquered by a Germanic tribe (weren't they Marcomans?) and finally Slavic people came and settled there.
there is not a general "Polish look", but I can tell some Poles from 100 metres away
I think it's not only "look", but probably something else as well, perhaps body and facial expression. I had a similar experience in London many years ago, at a time when there were very few Polish people from Poland in England. One black man who was trying to sell me a religious book said to me that I was Polish just after the few phrases I exchanged with him in the street.
At this point I am able recall the couple of my Dutch friends who came to visit us in Poland passing through Germany in their camper shortly after the German unification. They said they could never tell if someone was an Ossi or a Wessi while they were meeting them both on camp sites in Germany. But what used to amazed the Dutch couple most was that the native German man/woman was always right at guessing who a fellow German was: Wessi or Ossi. I wonder if this is still the case
