grammar - how do we tell if ばかり means "about" or "only" ?

If ばかり could mean approximately; about; and could also mean only; merely; nothing but;, then how should we know if this sentence ５０００円ばかりもっている。 means:

1) I have about 5000 yen.

or 2) I have only 5000 yen.

Answer

1. 
   

   ばかり after an amount or a quantity means 'approximately', 'about':

   
   

   
   

   ５０００円ばかりもっている。→ I have about 5,000 yen.

   
   

   僕はここ３年ばかりあの人に会わない。→ I haven't seen him for about three years.

   
   

   
   


2. 
   

   The sense of 'only', 'just' works after a verb:

   
   
   

   
   

   列車はたった今ついたばかりです。→ The train has just arrived here.

   
   

   
   

   or a noun/pronoun (not indicating a quantity):

   
   

   
   

   あの人は勉強 ばかりしていて... → All he does is study

   
   

   
   

Note that ばかりに, ばかりか etc. also exist and work differently.

As @Scott just pointed out, in your example, you would use だけ or しか (+negative) to mean "only".