If you can find out what the actual bobbin tension is supposed to be, this cheap scale here will help you ballpark it:
https://www.amazon.com/Manual-Postage-Kitchen-Mechanical-AMWHAND-BLK/dp/B003STEJ8O/
I'll attach the actual pic so that when that listing goes away, this post will still have relevance.
On domestics, about an ounce of bobbin tension gets me in the range and it's worlds better than guessing. If a person wanted to spend a LOT more money, actual purpose built bobbin tension gauges are out there. For our purposes, this little one works fine. It's a little fiddly, but who cares?