The phrase "difficult to determine" is correct and usable in written English. You can use it in a sentence to describe something that is not easily discovered or understood. For example, "The effects of the new policy were difficult to determine.".
The answers are difficult to determine.
The truth was difficult to determine.
That has been difficult to determine".
It is difficult to determine what, precisely, the labs do.
It can be difficult to determine refugees' ages.
He said that it was difficult to determine.
But officials said the number would be difficult to determine.
When I feel like I can't trust my brain 100%, Ludwig really comes in handy. It makes me translate and proofread faster and my output more reliable.
Claudia Letizia
Head Translator and Proofreader @ organictranslations.eu