The structure is slipshod, often forgetting key characters for long stretches while barely doing enough to justify the presence of others.  Rudd goes missing for a good chunk in the middle, and his scenes with Nicholson—who plays both Rudd's father and boss—exist only to move the story forward, but not the characters. 

That is to say one expects even in something as simple as this for Jack's curmudgeonly (and corrupt) old man to soften at some point, maybe even drop a bit of wisdom.  Sure, those too would be cliché, but at least they'd be attempts at dimension.  Not so here as it's an entirely one-note performance that ends with a whimper. 

Perhaps most surprisingly for Brooks is the total absence of wit.  The humor is soft and easy, with absolutely no bite.  Conversations are too meta, consisting of dating "rules", rights and wrongs, etc.; the so-called banter lacks any chemistry.  Opportunities for true satire are also lost (Reese's reliance on Hallmark-shallow inspirational quotes, for one). 

Likewise, character-conflicts should resonate more; the betrayals Rudd suffers early on should hurt, deeply and personally, but his response is merely pathetic.  Consequently, the humor doesn't deliver (despite all the attempts) and the stakes are never felt (despite how they're stacked).

Rom-coms of the past few years have been particularly stale, as if all churned out by the same studio assembly line, right down to the forced multi-ethnic casting of minor roles as seen here (African-American confidante, Indian girlfriend, Jewish psychiatrist, and so on).  Nothing else is taken seriously, so why should diversity?  How Do You Know is no different than the rest, as indistinguishable as them all; a particular disappointment given the pedigree, and a letdown in light of its holiday release. 

Even the answer to the titular question about falling in love is uninspired; in so many words, it's basically "you just do."  The same can be said for how you know this movie is a dud.

CAUTIONS:

  • Drugs/Alcohol:  Mixed drinks are made and consumed, as is beer (Guinness specifically, an obvious sponsor).  Drunkenness occurs, for comic effect.
  • Language/Profanity:  In an early scene, Nicholson spews three "f" words in succession.  Slang for male genitalia.  The Lord's name taken in vain once.  A few "s" words.  "P"ed off.
  • Sex/Nudity:  Couple in bed together, in the morning.  The sex they had is discussed.  Sounds of a couple having sex are heard, followed by seeing them immediately afterwards, under the covers.  Casual talk of sleeping around.  A man fondles a woman's breast at a party.
  • Violence/Other:  Nothing beyond comical hijinks (man falls down stairs, and so on).