You can tell the difference between the thread pitch, diameter, and length of the bolt. When it comes to grade, you can tell by the markings on top. The bolts shown are easily identified as grade 1 or 2 because there aren’t any marks indicating they’re any stronger.
These bolts can handle about 60,000-74,000 psi. Grade 5 bolts will have the 3 marks on the top. These are pretty typical for automotive application and have a strength of around 120,000 psi. If you need something stronger than that, you’ll need a Grade 8. These bolts have 6 marks on the top. The other markings on the head aren’t relevant, usually just markings of the manufacturer. Grade 8 bolts have the strength of about 150,000 psi.
Stainless bolts have the same strength as the grade 8 bolts. A lot of people think they’re stronger but they’re actually not. They do, however, resist rust better.
A grade 9 bolt isn’t very common but has a strength of about 180,000 psi. Unlike the others, it actually has 9 marks on it so you can’t mistake it.
Metric bolts are a completely different animal. They’re graded by a class. A class 8.8 is very similar to a grade 5 and a 10.9 is similar to a grade 8. To measure bolts you can use a bolt-sizing gauge.