Defending Against Damaten?

Let me say this straight out: It's not easy to do.

You can never be certain whether or not someone's in tenpai with a closed hand unless he or she declares riichi. Damaten is one of the unpredictable things in riichi mahjong. That's why people do it in the first place. If fact, a previous post of mine tells the pros and cons of staying in damaten. 

Disclaimer: If you're going to try to defend against a possible damaten, you should accept that you will bail out and likely will not win that hand. Things like breaking a hand to discard a "safe tile" when someone turns out NOT to be in damaten can and will happen. It's up to you to make the decision whether or not you will continue on the offensive or play defensively.

Anyway, so what now? 

Well you will never be sure when someone is in damaten, but you can probably infer from clues and make an educated guess. I already said it earlier, but inferring that the person in tenpai will not necessarily mean that the person is indeed in tenpai. It's poking at probability, but sometimes you might prefer playing defensively over offensively.

What are the possible "clues" I can look for then?

Essentially it all boils down to your opponents discards. That's right, you have to pay attention to their discards even if they don't call. In fact, if you want to be as sure as possible, you have to observe how fast they discard as well. 

Constant tsumogiri 

Tsumogiri or discarding the tile you immediately drew (most web clients like Tenhou show this). If the player keeps on doing this, and in quick succession as well (i.e. upon drawing, the player discards immediately), then it's an indication that one is in tenpai. In addition to damaten, this is one of the only tells you have for determining whether or not people who call tiles are in tenpai. 

The problem with damaten though is that people tend to gloss over their opponents' discards until he or she calls or declares riichi. 

Of course, there will be some cases where this tell would be too late. There are cases where players get ron'd immediately after one person gets into damaten tenpai. Just to keep in mind though, when someone keeps on discarding the tile they drew, you probably should weigh your options and consider bailing out.

Questionable discards

Alone, this isn't and shouldn't be enough, but if you were to combine this with the first tell, you probably can determine whether or not you should be wary of the person. 

For example, A discarded a 5-sou akadora. Now, you have to ask yourself: why would he discard an akadora? Most people would keep akadoras until at least their hand has finished forming. The fact that A discarded an akadora means that his hand is in tenpai. Now, if he continues to tsumogiri afterwards, well the chances of him being in tenpai just shot up.

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So that's basically it. I'll list down additional tells I think also can indicate a damaten, but they probably are not as reliable/more situational/only applies to RL. If you want an additional explanation, you're free to contact me.

Hesitation prior to discarding followed by a tsumogiri 
Discarding two out of three of a set in near succession towards the end/middle of the game
Personal tells (i.e. habits your opponent does after getting to tenpai)

Chombo-王

@nevertenpai