Abroad I used xe.com to check the exchange rates, but here it seems that local sources provide slightly different numbers than international exchange platform, and at the end of the day, we exchange money locally, so this is where we should be looking at.
These are the resources I check:
1) Banco La Nación (national bank of Argentina) - under Cotización billetes: http://www.bna.com.ar/Personas
2) Ámbito financero (financial newspaper) - https://www.ambito.com/contenidos/Dolar.html
3) Banco piano (exchange house) - https://www.bancopiano.com.ar/
For example, as of now (Aug 14, 3:30 PM ART),selling USD for ARS you get:
1) 1 USD = 57 ARS
2) 1 USD = 57.45 ARS (oficial) and 55,50 (blue)
3) 1 USD = 59.75