Since conditions can be stored in text, you can do it like this:
topic response
"accuse" "[if the evidence is handled]Yep, you got me.[else]I'm innocent! I swear![endif]"
But you can also do it like this:
topic condition response
"accuse" "[if the evidence is handled]1" "Yep, you got me."
"accuse" "1" "I'm innocent! I swear!"
And then check if the condition entry is empty.