How check if phone is sim unlocked?

Hi i exchanged a oneplus-one with a lg g3 16gb (sadly but still is fine, and with a JDI screen sadly but hey i came from a oneplus with a JDI screen so im used to yellowish) and im just curious to see id its realy unlocked the guys sayed its unlocked but he didn't check it.

And i don't have a other operator sim athome.

Is there a app or code to check it?

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