I think that after many years of watching the same riders, some commentators start to get some cues from face/respiration, body position and cadence, they may tell if the rider is cracking or not. The top riders can usually mask this signs, it's their best interest that their adversaries doesn't know how they are feeling.

Edit: These cues varies from rider to rider.

Edit 2: A rider can also use this to their advantage and misguide others: https://www.youtube.com/watch?v=9ea5fWA23Dc :)

The I.V. falls out of there arm.

Testosterone patch slips out of bike shorts.

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Probably when everyone starts passing him. I remember when Contador bonked at Paris-Nice, it was like he was going backwards through the peloton

Besides getting dropped?

They may sit up, start standing/sitting a lot, squirming around, cadence changes, start looking back, facial expression, shoulders slumping, etc., etc.

Usually they slow down a lot, too (and get dropped).

A tale-tell sign with me is me getting off my bike and pushing it up the remainder of the climb...

I loved that. It's featured in the old Sufferfest Angels Video. It feels like I'm riding faster than him as he drops backwards

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When you see them reach into their suitcase of courage only to find that there's nothing there.

Not sure I ever noticed it - but that's likely because I am busy cracking as well somewhere in those 8-minute segments

One of the best recent examples is Froome riding up Alp de Huez in 2013. He had richie porte ride back for food, which cost richie porte his GC placing (if I remember right). Pretty incredible stage!

Watching the shoulders is generally a pretty good indicator. Most riders are pretty still and composed on the bike, but when they do crack you start to see the hunched over and rocking side to side. "ankling" is a good indicator in more amateur riders, they just lose the form in the turnover in the feet. great sign they are suffering.

The tops guys have good poker faces, and they'll hold a position even if they are suffering. But when they truly crack you start to see those tell-tale cues that give it away.

Or watching Froome in the 2014 Vuelta. The commentators both said that he was done...as he dropped back from the rest of the GC contenders on the last climb of a few stages. Completely out of sight from the camera. Then a few kms later, he's just motoring back up to them, and right by them. Froome did this several times during that Vuelta, claiming that he was going to ride the summit finishes more like a TT and ignore what everyone else was doing.

His plan did work for the most part...and it was some of the most unbelievable riding I've ever seen since most times getting dropped on a climb just breaks you mentally.

I agree that Froome was cracking in the instance quoted above...but everyone thought the same thing was happening to him at the Vuelta last year. Turns out dropping back isn't a sure sign of cracking.

Gold!!

Also you need to watch the wheels closely, typically when someone is on the verge of cracking you'll notice they start yo-yoing a little off the wheel they are following. When the pace is on at the business end of a climb they should be locked on to the wheel. When they start to lose it a little then inch back up to it over and over they are probably close to their limit

Watch the feet. They will crack shortly after they start peddling squares.

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I remember watching a stage one time when Bob Roll and somebody were doing the call. A rider cracked and they erupted! He's cracked! It looks like he's going backward! His wheels are glued to the pavement!

I had to laugh out loud at their reaction. :-)

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By then Porte was already way down on GC. He lost all his time on stage 9 (the one where Dan Martin beat Fuglsang and Froome was isolated)

When Phil&Paul tell you the riders have cracked then they've cracked. Easiest way to tell.

Look at the 4th stage I think of Andalusia this year. When Froome attacked and Contador couldn't answer. The look on Contador's face, the body language, that's cracking. Contador is usually very animated; when he becomes subdued, he's cracked.

On the other hand, Froome always looks like he's cracking, even when he's kicking your ass. He all elbows flailing, head bobbing and weaving, and facial expression of someone eating a shit sandwich.

Love me some vintage Sherwin...

You know a rider is cracked: When the bridge radios to the engine room "more power!" and the engine room radios back "are you kidding?!"

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Actually, the exact opposite. Those two fools are senile. They get it right maybe 10% of the time.

In mountain biking its when they get out of the saddle.

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