I want to say something like "You can't announce him a cheater because it's speculation." I know that announce is the wrong word, so I need something to replace it. I thought of proclaim, but I don't think it sounds right, either. Help?
The phrasing you're looking for is...
Note that you need to include "as" in such constructions. In principle you could just use pronounce (which doesn't need "as"), but that's far less common, and sounds a bit dated/formal/stilted to me.
Also note that cheater wouldn't normally be used by competent speakers in this way (it is on the increase among younger people, but it's still mainly a marker for "non-native speaker"). Given that denounce is a relatively "up-market" word, you'd be more likely to see it used with, say, fraudster, swindler, trickster, etc. Or just plain cheat (in short, anything but cheater!)
As pointed out by @Jim in comments to the question, in popular parlance today you'd probably be much more likely to hear call or label, rather than denounce. And as pointed out by @Peter below, cheater is far more common in US than in UK usage (and it's a regularly-inflected form), so my carping about it should be taken with a substantial pinch of salt!