The defending champions head over to the Totally Wicked Stadium looking to break the last remaining 100 per cent record in Super League.
Impressive Saints have won all five of their games so far and McDermott said: “Our performance over at St Helens will tell us a little bit about who we are.
“Each year you find out and each year it’s a different story.
“We all talk about progressing year on year – how sides evolve and mature – which they do, but you have to understand each season is a story on its own, too.
“You have to go through – and I wish you didn’t – certain errors and moments in the year before you learn from it. We have to go over and put a good performance in at Saints and hope we show up well.”
Leeds, who beat Hull FC last week, have won three of their first four games as well as losing the World Club Challenge in Melbourne.
They will be challenged again this evening, though, especially without injured Australian Mitch Garbutt who becomes the club’s latest prop to be sidelined.
With him set to miss up to three weeks, McDermott admits his dwindling supplies of front-row players is a “concern” approaching the hectic Easter period.
Youngster Cameron Smith, 19, is set to get a rare chance tonight while, at the other end of the scale, veteran forward Jamie Jones-Buchanan makes his 350th Super League appearance.
Ex-England winger Tom Briscoe is in line for his 100th game in Leeds colours.
On Justin Holbrook’s dashing Saints, McDermott added: “They’re a very aggressive team in defence and seem to have found some rhythm in offence. (James) Roby running out of dummy-half with everyone flooding through has been a trait of Saints since Tommy Martyn’s been playing.
“They have a similar type of style there now yet very effective. It’s almost like they’ve woke up a little bit in attack. They are very good and then you put (Ben) Barba out of the back and they become extremely dangerous.”