1. Elder Kai statements in manga (even more explicitly referring to powerlevel in the anime)
2. Majin Buu's statement in the manga that they couldn't beat him even if they fused (he knows the dance, but not the potaras)
3. Vegito's statement in the manga that he didn't expect to be this powerful; Goku knows his and Vegeta's strength, and he knows how much the fusion dance increases power, the fact that he was surprised shows he didn't expect potaras to be that much stronger
4. The power difference between Vegito and Gotenks which can be roughly judged by estimating the powerlevels of Goku/Vegeta vs Goten/Trunks and weighing that against Vegito and Gotenks (Vegito more above Gotenks than Goku/Vegeta is above their sons)
5. Daizenshuu #7: "To use [the Potara], the two people who will merge simply have to each take one of the two earrings and put in on their left or right ear, respectively. Furthermore, after merging, the power is greater than with Fusion".
6. Dragon Ball: Super Exciting Guide claims the potara is a higher power multiplier than the Dance