55 year old toilet has crack in back of tank. Home warranty states crack is not normal wear and tear. How can a plumber determine that? That bathroom is rarely used and has not been worked on in last 10 years or so. Is it true the older toilets have more concrete in the manufacturing then than today?

Nothing that old is covered by any warranty, so 'normal wear and tear' is moot.
If this is a home warranty that came with the recent purchase of the house, then it should have been noticed upon inspection. At best you can sue the inspector, if very recent. Other than that (and that's a long shot, because it could have happened since the inspection), you are out of luck. It doesn't matter what the clay was mixed with to make the toilet, because toilet quality can vary, and all toilets can break for all sorts of reasons, often tightening a bolt, but even heavy people or freezing water in an unheated room.

See this web site for brief description of what toilets are made of.

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As far as I know no concrete is or ever has been used to make toilets. I toured the Kohler factory once. They actually do make sand molds then pour in porcelain clay and heat to make sinks and toilets. Think about that. A sand mold is made and then destroyed for every sink and toilet that is made. Very interesting tour but I didn't see a single job there that I wanted.

I wouldn't call a crack in a porcelain toilet tank "normal wear and tear". About the only normal wear and tear to a toilet would be the wearing away of the glazed surface. All working parts can be replaced.

A toilet tank may or may not get a little brittle after 55 years. If so, vibration or someone leaning against it may cause it to crack. If the glazing wears away such that moisture could seep into the interior of the porcelain and then it was subjected to freezing temperatures it might crack.

But, "normal wear an tear", no.

" Home warranty states crack is not normal wear and tear." This would lead me to think they should then cover it. They do not cover normal wear and tear. Is it leaking

A crack,is normally caused by some damage, But, I would not believe any home warranty would cover one, and esp not on one that old.

A cracked toilet tank may be an indication of some other issue. I have seen tanks fracture as the result of water hammer ( up to 2000 lbs of pressure ); sometimes caused by a quick closing ballcock. A water logged water storage tank will increase the potential for water hammer, as will pressure fluctuations sometimes found in municipal water systems. Porcelain is an extremely hard, durable material, and, unless the toilet was subject to some form of misuse, I would make sure to investigate all possible causes for the cracked tank. If you discover a cause for the effect, the home warranty may cover repair costs.