The phrase "a sure indication that" is correct and usable in written English.
It can be used to express certainty about a particular sign or signal that suggests something is true or will happen.
Example: "The sudden drop in temperature is a sure indication that winter is approaching."
Alternatives: "a clear sign that" or "a definite signal that".
"Traffic will be terrible," he complained, a sure indication that Federer knows New York better than it knows him.
His personal space was cramped by reporters — a sure indication that Stephon Marbury's travails are far from over.
Islamist parties secured about two-thirds of the vote in the first two rounds — a sure indication that Egyptians aren't interested in a secular state.
Not a single career lawyer signed the solicitor general's Supreme Court petition, a sure indication that something isn't right in the office.
Again it is very important to understand there is not a clear causal relationship, even when an ear or stalk mold is present it is not a sure indication that mycotoxins will develop.
Such offerings were usually quite sound but the fact that I had to read them several times to understand their structure was a sure indication that they would present an unfairly tough challenge to the solver.
But without any change in the prior law or any sure indication that Beck's 'law' is the law of the future, the State of Washington in convicting Beck applies special and unfair treatment to him.
If he says yes, that's a pretty sure indication that he's interested in you, or at least that he really enjoys your company.
Just about every university in the land appears to be planning to levy the full £3,000 a sure indication that the figure has been set too low.
There has been no sure indication that the miners are alive.
The interest in sustainability finance I am seeing by investment professionals and my own students are sure indications that we are in the midst of a meaningful economic transition.
