Oddly enough, a comma and a period look pretty distinct from each other unless your vision is bad. But for the moment, I will assume that you suffer from a condition known as "Punctuation Blindness" and you are completely unable to distinguish between the characteristics of decimals and commas. There is and has always been a way in this game to tell if that little critter separating numbers is a decimal or comma...
If it is a decimal, it will ONLY have TWO numbers after (to the right of) it. If it is a comma, it will ALWAYS have THREE numbers after (to the right of) it.
So, remember when you are trying to navigate the dangers of the GTN, THREE numbers after is a comma, and TWO numbers after is a decimal.
EVERY SINGLE TIME.