Sorry about your dad.
What happened with my own dad was we noticed the forgetfulness, and basic things he had done every day for decades he started having problems with. He was literally forgetting how to make a cup of tea, how to use the microwave, how to get dressed.
Dad's GP asked him to draw a clock face with the hands pointing at 2pm, he then gave him a name and address to memorise, asked him who the current prime minister was, what season it was, few more similar questions, then asked him to repeat the name and address he'd asked him to memorise.
We got the referral to the memory clinic but in the meantime there was blood tests and also a brain scan.
The memory clinic was more questions similar to what the GP had asked, also asking Dad about his childhood, his first job, his wedding etc.
The diagnosis was pretty quick, and what we had expected, dementia. The clinic prescribed medication to try and slow the decline.
Having said all that, depression, and also things like urinary infections, can cause memory and cognitive issues so it might be something that needs a course of medication, but isn't necessarily worst-case scenario.