Use the strength markings to determine standard to metric.  Many standard nuts have dots (on nuts), or those "lines" stamped into the bolt heads, while metric nuts & bolts are marked with a number.  The greater the number, of dots, "lines" or higher the number, the greater strength of the nut.

What you may not know, is the reason for specific torque values.  Making a nut/bolt too tight will weaken it's strength, which can cause  it to break, while too loose will cause it to eventually fall out.

Grades 0-5 are usually used on Autos.

All bolts, whether standard or metric, are sized according to diameter, thread pitch, and length.  A standard 1/2 - 13 x 1 bolt is 1/2" in diameter, has 13 threads per inch, and is 1 inch long.  An M12 - 1.75 x 25 bolt (metric) is 12mm in diameter, has a thread pitch of 1.75mm (the distance between each thread), and is 25mm long.  Though the two bolts seem identical, and are easilly confused, they are not interchangeable.

It should be noted that many fasterners, especially Grade 0-2 have no distiguishable markings on them.  You would have to measure the diameter of the head, thread pitch, and length, or compare it to a fasterner of the same size.