It is slightly easier to spot if you use GRCRTools, actually. It allows you to see the cost of the lost units on each side in your battle report. So if you attacked a city and killed 100 slingers and got 120 BP, the BP would come up as 120, but it would show that you killed 100 enemy population.
This wasn't during the BP days, but it was the first report I could find.
So you can see that I got 60 BP, from the standard BP icon in the middle. However at the bottom, you can see the population costs of the lost units, telling you that I lost 12 population, while the enemy lost 60 (this is an attack report - for a defence report, your population loss would be on the right, next to your troops). Now, if the population loss for the enemy doesn't match the received battle points (remember to account for Battle Experience with your losses, if you have it researched), then it means you are getting extra BP somehow. Unfortunately this doesn't work for support reports, or attack reports where you lose the battle.
I do agree that an icon is a good idea, but I hope this helps you to spot it until they (hopefully) add one.
