civilperson - what do you do if you don't have 30 breaks?
I am presuming (maybe incorrectly) that the asker is not from North America or developed country but that the mix design is being done on a project basis - not from a running batch plant operation.
Normally, you will design a mix, say 30 MPa, to have a mix design break at 30 MPa + X MPa - usually 8 MPa or so at this strength level. This is the target strength. When in production, the mix can be adjusted to reflect the statistics of breaks. In remote areas, we don't have batch plants and many times the aggregates are first time use for aggregate.
I would say as a minimum:
1. Make sure you have the mill test certificates for the cement used in the trial mix. Also, if possible, carry out confirmation tests of the cement.
2. Make sure that the aggregate properties have been included in the mix design documentation. Gradations of each part, Los Angeles abrasion (or Micro-Deval), and other tests presribed for coarse and fine aggregates by your particular specifications (ASTM, IS, JIS, etc.).
3. Make sure that you have with the documentation the required water tests to confirm that the water is suitable.
4. Make sure that the mix design computations (as per ACI or BS or IS) are correct.
5. Review the concrete break strengths. Again, you will have a target strength greater than the specified strength. If the aggregates have been used successfully in the past for concrete, the target strength will be lower than if they have not been. The target strength will also depend on the "level of control" that will be used in the concrete production.
6. Get a good book on mix designs (Neville and Brooks or Shetty, say) - they have good chapters on the concrete mix design and how to do it.
7. Make sure you are satisfied that the results can be reproduced and seem reasonable (and not cooked).
Most specs I have been involved with indicate that the contractor is responsible for the mix design he will use. You are only looking to say you have no objections or adverse comments to the proposed mix design. Your "approval" does not relieve the contractor to produce.
If you are not comfortable with making the "decision" - then do your best review and then ask help of a more expereienced individual. But, it will look good on you if you can show him what you have, where you are unclear, and what help you want. Not just pass the ball to him.