The Three Lions are top of Group I having won all five of their games so far.
England’s last two outings ended in 4-0 wins but Southgate expects a tough challenge against second-placed Poland.
“We are in a good moment and the team is playing well,” he said.
“We have a squad of players, any of whom are more than comfortable to play and are playing well.
“But also those moments are dangerous because, if we get any sense of complacency, we are going into a game that will be a challenge.
“It is a high level of motivation but also guarding against a feeling we are better than we are. We are getting a good set of results because we are working hard for each other.”
‘We have huge competition for places’
Southgate will also be buoyed by the fact the recent run of good results has come despite him making a number of changes.
The 51-year-old named completely different starting 11s for the recent games against Hungary and Andorra, and both ended in comfortable 4-0 wins.
Southgate believes that highlights the strength in depth England currently possess.
He added: “You could argue over what is our strongest team but the team that played in Hungary, we were obviously able to rest those guys after the game.
“There is still huge competition for places in this group and it is not so straightforward to say we won that game and this team will go again and give the same level of performance.”England beat Poland 2-1 at Wembley in March but the visitors were without record goalscorer Robert Lewandowski for that game.
The Bayern Munich striker is back to face England this time, however, and Southgate is aware Poland will pose a different test with him in their side.
“Any team is going to miss a player of that calibre,” he added. “We know he’s one of the best finishers in European football.”
Southgate believes Lewandowski and England captain Harry Kane display similar qualities.
“They are both very good at holding the play up and bringing team-mates into play,” he told BBC Radio 5 Live.
“They are both outstanding finishers and there aren’t that many old-fashioned number nines, if you like.
“In fact, neither of them are old fashioned in the way they play because of that ability to link and come deeper and play passes in behind, but I suppose a number nine is a focal point of the attack, rather than flexible forwards that might play wide at times or might play as an inside forward.”