Can you diagnose coolant leak in a refrigerator by blowing hot air on the back?
This is amusing.  No, I cannot think of how a hair dryer could be used as the key diagnostic tool in determining a refrigerant leak in a refrigerator.

That said, I have used a hair dryer on several occasions to accelerate removal of ice where a "frostless" fridge had a malfunctioning defrost cycle and ice had accumulated on the coils (caution: do not try this unless you know to avoid potentially heat sensitive parts).  Certain vintage GE units were notorious for this about 10 years ago.  Those units had several temperature sensors that would go bad, failing to regulate the temperature and activate the defrost when required and also they had main control boards that would go bad, failing to run the condenser fan as required.

The common signs of a leak are:  (1) The unit is low on charge, causing poor performance overall and the evaporator coil very rapidly accumulates ice progressively from one end until the whole coil is coated, and (2) trace amounts of oil around the area of the leak, although this is sometimes impossible to find.  To find such a leak these days, one would commonly use an electronic leak detection tool.

If someone knows how a hair dryer can be used for diagnosing refrigerant leaks (other than melting ice to see the coils), then please post it here so I can read it.

Edit:  If the fridge cools well and heavy frost takes several days or weeks to accumulate on the coil, then this could possibly be due to a failed electric defrost coil.  These can be checked with an ohm meter and are typically not too difficult to replace.

Edit 2:  It appears the SMEG FB30 is not equipped with an electric defrost coil and it merely shuts of the compressor to melt accumulated frost, so my comment above is not applicable to your unit.