The best thing I have found for my piece of mind is to candle my eggs. I use a Photon light that we use out in the field to read our maps or send signals.
Here is a link: www.cfrlights.com/photonlight.html
I would suggest getting the white light. These lights are small powerful and you can actually sink them in the substrate and see all portions inside the egg without ever having to move the egg itself.
I usually wait a week after placing the eggs in the incubator. If the egg has a reddish glow, or you see any pink at all (a pink circle, a vein) the egg is fertile. If you see a yellow greenish glow and no pink/red at all, it’s probably infertile, BUT since it is unpredictable Mother Nature we are dealing with I usually place the "green" egg in a container away from my pink eggs. This is because if it is unfertile it will most likely start to mold, and I don't want that to possibly affect my good eggs. Maintain your temps and humidity.
Candle your good egg, don't move it and see if the entire egg is dark (baby Leo) with a little tip of pink. If so watch closely, if the egg starts to sweat and dent and no little head emerges after 24 hours you might have to help the little guy out by cutting the egg just a little bit.
Hope this helps,
Tim

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"I'd still rather spend the day with an animal than most people I know"
LEOs: 6.9.0
1.3 High Yellows-Barney,Betty,?,?
2.0 Hypo Tangerines-Bam-bam,Rex
0.3 HCT's -Journey,Wiggy,Lil'bit
1.1 SHTCT's-Fred and Wilma
1.0 Rescued Normals -Shleprock
1.3Tremper Albinos -Mr.Slate,?,?,?
1.0 Tang Tremp Albino - Rocky