How to differentiate normal potato from the potato that has sweetness?
Though there is not a perfect way you can differentiate them except tasting, but you can try following ways which are kinda based on experience as we used to run a grocery store.

1. By smelling them

You can smell them while purchasing. If you smell some sweetness, it will have sweetness. Though, it'd require some practice.

2. By texture and color

You can differentiate between them by looking at color.

For eg., this one has sweetness.



While this one doesn't have.



There is one more, but it works after boiling.

If you boil potato with sweetness, it will get glue-ish.