Does a presence of ants on a plant mean that the plant has been infested by aphids?
Simple answer is no:

$Ants \neq Aphids$

However, they could be used as evidence that aphids are there depending on the reliability of the coexistence. Ants and aphids are independent entities and do not rely on each other so can exist in non-overlapping populations. Applying this idea to the correct species of ant and aphid is, of course, important for it to work as an indicator - it will only be true in those combinations where this "aphid protection" occurs.

I can't find any information on the reliability of the relationship, especially not knowing which species you are looking at. If there was a high positive correlation of ant density and aphid density then one would expect the presence of ants to be a good indicator to the presence of aphids. If the correlation is low there is a higher chance of a false positive (where a false positive = when you see ants, conclude there must be aphids, and there are no aphids).