You may write a short parser that checks each character:
LPCTSTR lpszParse = strDate.GetString();
while (*lpszParse && _tcschr(lpszParse, _T("01234567890/")))
lpszParse++;
if (*lpszParse == _T('\0') && !strDate.IsEmpty())
{
}
But with a fixed date format you may also parse it completely to check for a valid date:
int nDay = _tstoi(strDate.GetString());
int nMonth = _tstoi(strDate.GetString() + 3);
int nYear = _tstoi(strDate.GetString() + 6);
if (strDate.GetAt(2) == _T('/') &&
strDate.GetAt(5) == _T('/') &&
nDay >= 1 && nDay <= 31 &&
nMonth >= 1 && nMOnth <= 12)
{
}