How can I find out if someone has been fined for not paying enough tax?

An ex employer of mine has said he is being investigated by the Inland Revenue for not paying enough tax, and they said he has to pay 拢3000 back, how do I find out if this is true.

It is supposed to be my tax.

This wouldn't go through a court, it would be "assessed" by tax office, and done through there.
If HMRC have told you liability is being charged on employer, you don't have to worry, even if for some reason (like bankruptcy) he hasn't paid. The charge for your tax is levied on him, not you. This relieves you of the liability. HMRC will not tell you details of what ex-employer is charged, or what he has to pay, that is subject to the Official Secrets Act.
The only way this could affect you is if, for example, you wanted to claim expenses in employment, and ask for a tax refund. As you did not pay the tax, no refunds can be given out of it.

don't think you can unless he's not been paying your tax

You can't find out - everyone's tax affairs are private and HMRC would never reveal this info. How would you like them to tell other people about your tax affairs?
Its quite possibly true - if your employer failed to pay over the PAYE he was deducting from you, then HMRC would expect him to pay, plus interest and a penalty.

I would imagine this kind of information would be confidential. If he was tried in a magistrates or crown court then you would be allowed to watch from the public gallery

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