As already said the primary check against abuse of a pardon is the threat of impeachment.
However, I'd argue there is another, unofficial, check against presidential abuse of pardon that is worth discussing, the states' judicial systems.
I already go into detail about this in another question, so I won't repeat myself too much. The short version is that most federal crimes are also state crimes, and thus in most cases anyone pardoned of a federal crime usually could still be convicted of a state crime for the same action. The supreme court has ruled that this is not a violation of the fifth amendment's protection against double jeopardy since the state and federal governments are separate entities.
There has been a long running common agreement between the state and federal governments to respect each others decisions in regard to a crime. If someone is put on trial for a federal crime the state will generally respect that ruling, rather the person is found guilty or innocent, without having a second state level trial; and likewise the federal court's general respect the state's trials. This is NOT constitutionally required, but it's still a very strongly held tradition in deference of avoiding double jeopardy.
Generally this has also been true for federal pardons, state's respect the pardon and do not prosecute the pardoned individual of the same crime on the state level. However, there is no legal requirement for a state to do this. If a state feels that a pardon has been abused in an unjust manner they have the right to convict the pardoned individual on the state level, assuming they have an appropriate state law that was violated. This would require breaking a very strongly held tradition, and as such would likely only be done in the event of very blatant abuse, but it still stands as a second line of defense against such an abuse by giving a means that a pardoned individual may still face justice for his or her crimes if the pardon is agreed to have been abused.