Is it if the keel bone is SHOWING, or if you can feel it when you touch their chest? I don't have a Lentil-appropriate scale in the house, don't feel like taking him to the vet, but my fiance says he's looking pudgy. I think he was 103g when I took him to the vet a couple months ago and I haven't noticed much of a change, but he DOES get treats for a LOT of cute things he does, and his treats are nibbles of millet.
Oh, and I can feel his keel bone when I touch his chest gently, and can kind of see it depending on how puffed his feathers are.

If you can feel his keel bone and it is more prominent than the breast meat on either side then he is not to fat. Now if it were the other way around and he felt like the keel was sunk in like a river and a hill on both sides then he would be too fat. Course there is also too thin but yours sounds fine. I weigh mine but prob not as often as i should. I rely alot on just feeling them and keeping close watch on how much they eat and overall health and well being.

Thanks! That answers my question perfectly

Good description Kate. I was going to say if it looks like Lentil has cleavage he's too fat but if his keel bone is really prominant then he's too skinny.