There's a long list of coolants and the supported types by different manufacturers and it's virtually impossible to guess existing coolant in a system from a used car. The problem arises when topping up is required and most of the people of would simple flush the system and put in fresh coolant if they are unsure of current type. This video shows a tip to check if a new coolant can be poured in a system with existing coolant to top up without causing the formation of cooling Gel that's usually a product of coolant incompatability.