It's pretty difficult to distinguish what part of the date corresponds to month or day or year, when you use free text entry, but a good place to start, if you're looking for a specific format like you listed, is to use the LEN() function.

In this case, you know the desired format is DD/MM/YYYY which constitutes a character length of 10, so you could use something like;

SELECT * FROM YOUR_TABLE

WHERE LEN(DATE_FIELD) = 10

**OR: WHERE LEN(DATE_FIELD) <> 10

You'll probably have to manually sort through the "exactly 10 character" dates and update any yourself which do not fit the criteria...