From what I've read, grind issues are one of the major and common issues with all knives;
This isn'tt exactly true, yes they're a concern but not in a major or common issue, however, some makers are known to have major & common issues with (over) grinding. This is why I go on about Moritaka so much (versus every other maker) - because they're habitual bad grinding offenders whereas most others do a very good job. There are some exceptions like Aritsugu A-types - they suck grind wise as well. As for non-Japanese (American) knifemakers there's lots of over grinding going on here - 2" wide belts make for 2" wide divots.
...overground knives can be more flexible than correctly ground knives, can cause a knife to steer because one side of the knife is ground more than the other, and can cause a knife to not cut smoothly, among other things.
On flex - it depends because if a knife is over ground in a way as to be too thin then yes it'll be flexible but it's more likely that a knife will be thick and have overground section and be very stiff still.
If the sides are ground uneven then yes the knife can steer. This is very common with many Japanese knives that are purposely designed with uneven grinds from side to side. That's Ok for those knives though since they're purpose built and the user needs to learn proper technique to perform said purpose correctly. I believe most gyutos (even asymmetric ones) are not meant to steer though.
However, I'm not sure what to look for. I have knives that are more flexible than expected, knives where one side of the knife seems to be ground more (the grind seems to go higher than the other side). But, I'm not sure these knives are overground or incorrectly ground.
What are clear markers or things to look for to identify grind issues?
If a knife steers from flex then I'd say it's the wrong knife for the task or maybe just wrong for you.
Japanese knives are all ground asymmetrically - every last one of them so finding them ground uneven from side to side is to be expected. Is the knife ground correctly for the type of knife that it's meant to be - this is the question.
Clear markers for overgrinds are an edge bevel that goes through different thicknesses (along the length) like a roller coaster. A wide (at the heal) bevel that steadily decreases to very thin at the tip is normal but tall and short and tall again are clear indications that the knife has been overground in sections. The thin sections are the overgrinds. I suggest sharpening the knife upon receipt - if the bevel goes all roller coaster like then you just uncorked the maker's cover up. It's easy as hell for a maker to cover up an overgrind (looks great OTB) but it's way easier for you even to uncover it by using a flat stone.
Another clear marker is the low hanging heal or a hole in the edge that shows light under the edge in one spot or where another spot doesn't allow for the edge to make full contact with the cutting board.
I like to not only look at the edge as it rolls along the cutting board but much more accurately assess the board contact from eyeing down the length of the edge with the tip in my left eye (well almost) and cutting edge up. If you look down the belly (using your right eye) you'll see everything clear as day.
You can also go one step further and use an engineers square (straight edge - not a ruler!!!) to find highs and lows on the blade sides. Pay particular attention to the area immediately (1mm) above the edge bevel - this is the zone that'll cause you trouble when sharpening.
Hope this helps you some.
Dave