Code:
	BOOL IsDebuggerActive() {
		typedef unsigned long(_fastcall *bNtQueryInformationProcess) (IN HANDLE, IN int32_t, OUT PVOID, IN ULONG, OUT PULONG);
		bNtQueryInformationProcess NtQueryInfoProcess = nullptr;
		HMODULE Nt = LoadLibrary(L"ntdll.dll");
		NtQueryInfoProcess = (bNtQueryInformationProcess)GetProcAddress(Nt, "NtQueryInformationProcess");
		PVOID ProcessInformation = NULL;
		unsigned long Ret = NtQueryInfoProcess(GetCurrentProcess(), 0x7, &ProcessInformation, sizeof(ProcessInformation), nullptr);
		if (Ret == 0x0 && ProcessInformation != 0) return TRUE;
		else return FALSE;
	}
Credits to McAfee (website) and Axion