It depends on context. At one end we have:
"Mr Bond, they have a saying in Chicago: 'Once is happenstance. Twice
is coincidence. The third time it's enemy action' " -Ian Fleming
This might apply if what you suspect is happening might put your life on the line, and you have reasonable grounds to think someone might want to do that ("Never go against a Sicilian when death is on the line!"). In casinos people who seem 'too lucky' get kicked out and banned, even without proof of cheating.
In science they use confidence intervals:
The 'gold standard' for new physics is six sigma certainty. That's only a quarter of one percent more than three sigma, but for results that could potentially overturn lifetimes of work by others, or form the basis of the next generation's work, the difference can be crucial. Here's a nice article by Sean Carroll about an anomaly in coin tosses before sports games: A 3.8-Sigma Anomaly. Personally I'd still look at whether some sneaky way of biasing the result is involved.
Specifically in physics, it's not uncommon to reference 'unlikely in the age of the universe'. 100 heads in a row has a likelihood of order 1 in 10^30. There are 10^17 seconds or so in the current age of the universe, so even at 100 flips per seconds this result would be extremely unlikely to occur in fourteen billion years. So, it would be a lot more likely to indicate a biased coin.
The results where we can't rigorously model things is the Philosophical topic of miracles, and whether a given one violates Naturalism. We could actually do a pretty good model of water vapour interactions; you'd have to give parameters for how fuzzy the letters are, but you could get a fairly robust likelihood. It's going to be unlikely in the current age of the universe. But what exactly the consequences you take are, requires more thought. You'd probably look to Bayesian reasoning, and there are a lot of priors for Naturalism. You might think: it is faked somehow (other examples? Technological possibilities?); I am mentally ill (did others see? Photograph?); we are in a computer simulation (Are we living in a simulation? The evidence).
It's a lot more likely we see faces in clouds: Pareidolia: Seeing Faces in Unusual Places. Babies respond to human faces before they can focus their eyes, so we know there is a brain structure as well as learning involved. People really do feel things they see in clouds have significance, and this has been linked to religious behaviour:
Faces in the Clouds: A New Theory of Religion. Statistically, it's a lot more likely a human brain is 'a biased coin', than that we witnessed something unlikely in the age of the universe. At best, we might expect to infer deism, consciousness at some larger than human scale involved in the universe's setup, but with no evidence of interest in our daily happenings (eg, impacts of prayer - this is a whole topic).