The answer will depend on what country you're in.
Most US employers won't tell you whether an ex-employee was fired. The norm here is that they will tell you the dates when then employee joined and left the company, and won't say anything else at all.
Over the last fifty years or so, many people in the US have sued their former employers for giving bad references. In many cases, it's come out in court that the ex-employee actually had a good work record, but somebody gave them a bad reference because the ex-employee was a different race or religion or gender (which are protected under US law), wouldn't babysit their children for free, was angry because the ex-employee wouldn't have sex with them, etc.
It's not even safe to say good things about ex-employees here. If - for instance, - a company gave good references to white people but not blacks or Asians, they'd lose a discrimination lawsuit.
The damage awards in discrimination suits can be enormous, often running into the hundreds of thousands or millions of dollars per incident, as a little googling will show. Wal-Mart is currently facing an enormous gender discrimination suit that may cost them billions of dollars if they lose.
To avoid this kind of lawsuit, normal HR practice in the US is to say as little as possible about ex-employees, limiting it to simple facts with no type of judgment about the ex-employee. Even saying "we fired this person" would be a negative statement, so very few companies would say that.
Here's something that might work: if you could talk to their supervisor and ask "If this employee would apply for a position with you again, would you hire him". If the supervisor said "absolutely!" then you know you have a winner. If you hear "well..." then you know you shouldn't hire that employee whether he was fired or not.
Is this the previous job or several jobs ago? Ask them why they left in the interview. Are any of their references fromm that company? Where are their references from and do you have reason to believe they are not being truthful in their interview?