The proof of Juan Manual works perfectly but I want to explain why its works.
Some words (can be nouns, adverbs, etc) needs a preposition ("de") before the relative ("que"). The relative, as you maybe know, it's a referencial to the first element that you want modify with a proposition. I'll be more clear with an example:
Estoy seguro de aprobar.
If you replace "aprobar" (that works as a noun) with a proposition:
Estoy seguro de que aprobaré.
You must use "que" because it's the relative of the proposition.
So, both of the following phrases are wrong:
Estoy seguro aprobar.
Estoy seguro que aprobaré.
But, too many people in spanish don't "mistake" this. When someone really do a "queismo", if almost because is an overcorrection, they fix a good way to say it.
Returning to the topic, the real proof for an advanced speaker is replace the proposition by a functional word as some pronoun, but the real proof is split the main sentence and detect the proposition and the relative.