With an Alter Ego experiment.

Deciding to test something, that being whether Erza's mental state with Transformations would... Update, for lack of a better term with new information. For example, with her currently "Transformed" into Lucy, she was essentially programmed to imitate her to the best of her ability and knowledge, fully believing she was in fact Lucy. Would she in fact adjust her behavior upon seeing Natsu use some of Lucy's programming on the spot, or would he need to turn the Lucy Transformation off and on again for that?

Only one way to find out. "Alter Ego, Slave," he said to the real Lucy. Immediately, Lucy's demeanor changed.

"Master? May your humble slave know exactly what's going on?" she cooed, approaching him in a sultry manner with her hands behind her head.

https://imgbox.com/kcUL4tbx

"I'll tell you later," he chuckled, turning his gaze back to Erza, who gasped at Lucy, rather confused as her mind processed "herself" not acting like herself.