That's the issue that I'm having as well. I jumped on CoralPedia and I'm still not sure if they're eagle eyes or watermelons, for a few reasons. First, watermelons have an almost fluorescent green tentacle, while eagle eyes have a more greenish-yellow tentacle, so that's near impossible to determine unless, like you said, they're side by side. Second, eagle eye polyps have a pronounced green-tinted mouth, while watermelons are more subtle and blended in more with the oral disc (A few that are in my buddy's tank have a green tinted mouth, but the others have a blended mouth and oral disc). Finally, the oral disc on watermelon has a larger portion of orange blended from the mouth to the tentacle crown, while eagle eye has a narrower section of orange. Dunno...
Eagle Eye Zoanthids
Wam'n Watermelon Zoanthids