My husband-to-be is a foreigner. How do I make sure he is really not already married in his country?

Do not rely on his word alone; ask for an official document of his marital status from his country. The religious affairs office handling the marriage usually requires such a document, and it protects you from becoming the wife of a man hiding a family there. Besides documents, get to know his family, ideally in person or by video call with his parents. Ask open questions about his past and notice whether his answers are consistent. Involve your wali at every step. If he avoids, delays or gets angry when asked for proof, that is already an answer. For the exact documents required, ask the state Islamic religious affairs office where the akad will take place. See marrying a foreigner.