[c#] Check if boolean is true?

It depends on your situation.

I would say, if your bool has a good name, then:

if (control.IsEnabled)  // Read "If control is enabled."
{
}

would be preferred.

If, however, the variable has a not-so-obvious name, checking against true would be helpful in understanding the logic.

if (first == true)  // Read "If first is true."
{
}