gold barbs have an lifespan of 3-6 years average , i can tell they are happy as they grow, larger than what the average gold barb does, as well as this, they have coloured up alot, which they only do if they feel comfortable, they never nip which means they are settled, and are very interactive fishes, from all this i mentioned, i can deduce that these fishes are happy, although you are right, i can never be 100 percent sure, but ive never had even the slightest problem with these barbs also, 20 gallon long(24 US gallon) i feel is sufficent for 5 gold barbs imo