True deep cycle flooded lead acid batteries have three characteristics that set them apart from wannabe batteries: filler caps, more room above and below the plates for electrolyte, and heavier plates. Unfortunately it is hard to know about the latter two by looking and many manufacturers of 12V batteries slap a label on a starting battery and call it deep cycle. One sure fire indication is no amp hour rating on the label.
With few exceptions the only true deep cycle battery is golf cart 6V batteries. These are designed for a specific deep cycle application and any batteries that don't hold up in that service are quickly run out of that market.
The only exceptions in 12V types are very expensive like Lifeline batteries.
AGMs are different. As best I can tell there is no difference between starting and deep cycle batteries. Their design characteristics make the three criterion above meaningless.
David
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