How do you know whether two quantities x and y are proportional?

You can tell that quantities x and y are proportional if they are effecting each other.

Step-by-step explanation:

If two quantities are proportional that means they are dependent on each other.

The quantities can be directly proportional and inversely proportional as well. If the increase in one quantity will increase the other quantity and decrease in quantity leads to the decrease in other then it is directly proportional. Like in Ohm's Law: the current and voltage are directly proportional in linear circuit while neglecting the resistance. The following expression show this direct proportional relation:

V ∝ I

If the increase in one quantity results in the decrease of the other quantity or vice versa then these quantities are inversely proportional. Like if the people required to do a job increases then time required to complete the job will decrease or vice versa.