This is almost completely wrong: the number of significant digits is something given by metadata, you cannot look at a number and know that it has been properly rounded to the correct number of significant digits. You always need to be told this special circumstance because most numbers you encounter have in fact not been properly rounded.
.000300 you have 3 significant figures.
300000 you could have 1 to 6 significant figures.
3.00x10^5 has three significant figures.There is the odd case of trailing zeroes in non decimal numbers, but otherwise if a number is written, when is that not assumed to be significant?
That is, there are fairly standard rules for conveying significant digits, I thought?
For example, the number you are viewing could have been produced with finite precision floating point math and then just rounded to an arbitrary chosen (small) number of digits.