Hi there welcome to the forum Well I have to say having a tiel is definetly cool
but you should do your reading as its not for everyone, I am not sure who told you that lines over its belly means its a female thats totally false if its an all white tiel the only way to tell if its a male or female other then dna is to know what mutations the parents are there is no way to visually sex an all white cockatiel the only other way is to go by behaviour if its more vocal its usually a male but there are also females that can be vocal do you know how old the tiel is?