So, there are a ton of differences throughout the movie. Most are between the CGI and hero suit, but some are even different among CGI suits and the CGI/Hero amalgam. I'll just show a few.
First, note the weirdness going on with the helmet. There is a ripple present on the helmet that doesn't appear anywhere else in the film. Also note the upper middle trapezius area. It's mostly flat with clean, sharp lines.
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Now compare to this grab a few seconds later. Here we see no more ripple on the helmet, and the upper trapezius panel, while it looks very similar, seems to be larger on the top side than it was in the previous grab. The difference seems slight, but we'll compare it to a frame later on that is more severe.
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This grab is just before the Iron Monger fight. Here, most of the upper torso/helmet is practical, and we can see the big difference in the upper trapezius panel. Instead of the clean, sharp lines we had before, this clearly is more more rounded, like it was vacuum-formed plastic.
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Moments later, we see an entirely different trapezius panel, shorter than all the others. I'm pretty sure this is also practical, given the sloppy splitting of the trap/shoulder panels. Also note that you can see RDJ's jaw underneath the helmet. Some pieces are clearly missing here.
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Some seconds before this we see Tony suiting up to fight Iron Monger and the upper torso is a practical suit, as seen by the difference highlighted in the next two pictures compared with the third which took place before the Middle East fight.
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Now compare with the earlier shot and see how different the visibility is of the endotech on the CGI suit (below) and the hero suit (above).
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So lots of stuff. Most of it is a CGI/hero suit difference for reasons of practicality.