Is there any way to test what the real amps of a power supply are before buying from a store?

In a word No.
Even  if you have one in hand you can not really measure the amp rating.
Also there are a couple numbers to look at  things like generators and power supplies have a max amperage rating based on the maximum amps  for a short period time sometimes called peak amps. the other more practical number is continuous amps this is the power supply will really put out  without danger of overheating.

A power supply will only  provided the amps needed for a given load .
Not sure what you are really asking.