To finally determine how healthy the battery is, use of a hydrometer is essential. The hydrometer measures the electrolyte fluid's specific gravity, That is the indicator of how healthy the chemical reaction is within the battery and if the battery has reached maximum SOC (State Of Charge).
A good hydrometer costs around $20 and an inexperienced user would need to follow measuring procedures for safety and accuracy. Most folks won't try it since it involves battery acid but it is the definitive measurement. Hydrometers have saved me hundreds of dollars over the years.