If you can insert the phrase "in order" before the infinitive, then it's an adverb of manner, like this,Originally Posted by jozi209
He went to the store in order to buy some food.
In order to buy some food, he went to the store.
If you can't insert the phrase "in order", then it's not an adverb of manner:
To be honest, I don't know.
I don't know, to be honest.
*In order to be honest, I don't know. (ungrammatical)