Very good advice which I am going to start following. Mine are not that hard to handle but I have not been very systematic about examining them - I judge crop empty or full by sight - they go to bed with these enormous bulges in front and then in the morning they look svelte around the neck.If you are unsure about what a crop should feel like at a particular time of day, the easiest answer for you is to feel your other hens crops and compare.
The statement that a hens crop should be empty in the morning can be misleading if one is not familiar with how an empty crop feels. Often one still feel some content in a crop in the morning and this is normal.
So, check all your hens at night as they roost and again in the morning before you let them out.
Sour crop can be deceiving. Often the crop still functions to a degree but is often slower than normal.
I see you have read Two Crows article which is very good. The problem an inexperienced keeper has is often determining which crop problem their hen has. A partially impacted crop can easily be mistaken for sour, or slow crop.
I'm sure you will get advice on the ER thread.
My advice is always take the hen to a vet if you have a vet that is competent and you can afford the cost.
My other piece of advice is to get used to handling your hens. It will give you and them more confidence. The easiest way is to do this at night while they are roosting. A good headtorch helps. Take each hen off the roost and stand them in front of you preferable on a table. Do this in the dark bar the head torch and feel the hen. Check under their tail feathers around their vents, lift their wings and look underneath and feel their crops and down their keel bones. If you do this every night for a week you will learn more than hours spent on the Internet. What is normal for one hen may be odd for another.
I do hopeshe turns out okay.