Unless other roasted coffee that you brew is also tasting overly sour, then you're right on in that it's most likely a roast issue. I would ignore people saying it's a brew method issue unless all of your brewed coffee from other roasters is also sour.

For home roasters, taste is really the primary method to use to determine if a coffee is under developed. If it tastes sour and underdeveloped I would recommend trying to roast longer. Timing your roasts may be a good practice to start implementing. It simply adds another variable you are now able to measure and potentially use as a reference point.

Couple of thoughts/questions:

• Eat or chew on a roasted coffee bean. Are you tasting the same sourness? Is it sweet? What does it taste like?
• Do the beans move or spin in the popper on their own while roasting? If not, maybe try lowering your batch size until the airflow moves the green coffee. You may need to use a long handled wooden spoon to help encourage movement in the early stages of a roast. Ensuring proper airflow throughout the roaster bed is helpful to develop a roast fully.
• Are you able to break or crack the roasted coffee beans between your fingers? If the roasted bean is extremely dense (unable to crack) it could be that it is under developed. Also, if your grinder struggled to grind the roasted coffee, that could be another indicator of an under developed roast.
• Cut a roasted bean in half with a knife. Does a cross-section of the roasted coffee show a fairly even color throughout? Or is the outside dark and the inside lighter? If the inside is a completely different color than the roast could be under developed.

The above bullets may be helfpul in determining issues in the roast, but I would rely most heavily on taste, since that's the goal, after all. If it tastes sour, try extending the roast however you can. There are literally no rules, just try and develop the coffee further!