The contract must be in writing.
Except in rare cases involving fiduciary relationships and partial performance, it is almost impossible to enforce a verbal contract for the purchase of real property in California.
The plaintiff must have performed.
Whichever party sues to enforce the contract must have fully performed all required of him or her before bringing action, unless prevented by the other property.
The plaintiff must be ready, willing and able to perform.
If the party seeking to enforce the contract is not able to deliver all the consideration promised (money or title as the case may be), specific performance may be impossible.
The contract must be fair, just and equitable.
If the contract is in some way unfair, the consideration (price) grossly inadequate, or the sellers sick and elderly and with nowhere to go, for example, a court may refused to specifically enforce the contract.
It should be worth it.
Since all litigation can be expensive, time-consuming, and aggravating, with the uncertainty of winning at all, caution should be exercised before filing a suit for specific performance. There is always another house somewhere to buy, or another buyer out there to purchase, so unless it is really worth it, financially and emotionally, specific performance should only be a last remedy.