One diagnosis explained, now I'm confused if I have the other one too

Son diagnosed with autism. Now I'm reading about ADHD and going 'wait, do I have this too?' Where does one condition end and the other begin?

How do I know if this is two separate things or one puzzle with overlapping pieces?

If you have multiple diagnoses (or wonder if you do), how did you figure that out? How does it change your understanding of yourself?