There are several ways to prove that your English language skills are at C1 level or higher:

• Previous degree obtained in English or university entrance qualification from an English-speaking country
• TOEFL internet-based (iBT) or TOEFL iBT Special Home Edition: a total of at least 95
• IELTS (Academic) overall score 7.0
• Cambridge Examinations: C2 Proficiency, C1 Advanced at least 180 in total
• UNIcert III
• PTE Academic (also online): at least 76 in total

More information about proof of language can be find in this link.