The only option you have for checking the grade of steel while working at site is to look at the stamping on bar for its manufacturing details.
All ISI mark rebars manufacturer emboss the details of reinforcement steel on every reabr. the rebar do have following details emboss on it;
Dia of reinforcement bar
Brand / trademark
Grade of steel
To verify the grade of steel, you can inspect those details on steel bars randomly.
If you wish to verify, does steel bars exhibits the same properties of grade of steel mention on it, you need to test it on universal testing machine for tensile strength of it. if universal testing machine is not available at project site (in maximum cases, this machine is not available at project site) you can send the samples of rebar to third party testing laboratory which is near by you and acredited by NABL.
To know how to check the reinforcement steel bar when new lot is received at project site, below articles might help you to learn more.
Quality of Reinforcement Steel Bars used for Construction of RCC Structure. When a lot of steel received at site, First check the Manufacturer test certificate for its actual properties. With each lot of steel, manufacturer should send a test certificate of same lot for test done at their...
Types of test done on reinforcement steel bars Following tests are done on reinforcement steel bars from third party laboratory as per the frequency set for testing. Mechanical Test: Area Gauge length Final length 0.2 % Proof load Ultimate load 0.2% Proof Stress U.T.S % Elongation Fracture Bend...