What do we see that tells us it not a small date? There are several differences between the the two. First is the '9' is different on the large date and the small date:

The only small dates are on the 1970-S cents, but there are two different dies for that year. Note the above images. Can you see the differences? Note the curl on the '9'.
Also the location if the '7' is lower than the rest of the digits on the large dates:


If you are looking online for these, you might use this trick, a business card:

So if the image is rotated, the card shows the real deal from the want to be small date.
So the first thing I look for on these is the lower bottom of the '7'.

That is my first clue. Then proceed from there.
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1970-S small date. How to tell