To determine the strength of a bolt look at the head of it & count the # of raised radial lines. No lines is a soft bolt, use them to hold your fence or gate in place. Called low strength steel they have a tensil strength of 60,000 psi. Not something you want to use if you value your life.
3 lines is a grade 5 & is used as a general service bolt. You may want something stronger for suspension service. Grrade 5 has a tensil strength of 120,000psi.
Grade 8 is about the best sae bolt & has a minimium tensil strength of 150,000 psi. They have 6 marks on the head.
Stay away from stainless steel bolts, many are are as soft as the low strength steel ones.
Metric bolts are marked with numerals on the head.
8.8 is equilivent to an SAE grade 5 bolt.
10.9 is equilivent to an SAE grade 8 bolt.