How often do we read or write that we should check something? Example: "Check if the computer is connected to power." Clear message, no misunderstanding. But what if it says: "Check that the computer is connected to power."
What's the difference? Is there one? Yes, and it's not insignificant.
Check if versus check that
When I ask "Check if the computer is connected to power", I want to know if the computer is connected to a power source. Probably because it does not turn on. (And yes, I am also thinking of Chris' standard question now.)
When I write "Check that the computer is connected to power", then this could be part of a step-by-step guide explaining how to get a computer up and running for the first time. (Make sure the computer is plugged into the power supply before turning it on.)
"Check that" means "make sure that". When I write "check if", I may not be quite certain about what causes the problem.
By the way: Instead of "check if" you can also write "check whether". But it's more formal, longer, and it's difficult to type. I sometimes accidentally type "check weather", and then I have a problem if the spellchecker doesn't recognize the error.