You do it before you go to the field,
or
You ring the AIS information line on 0500 354802 (free from a landline) if you are in the UK or on 020 8750 3939 if you are outside the UK. That gives you all of the RA(T) and temporary airspace upgrades.
or
You ring an ARO and ask them for a brief (if you can find one)
or
You ring the AIS Duty Officer on 020 8750 3773 (H24) and ask him.
What you don't do if you want to stay within the law and avoid a prosecution under Part 10 of the ANO is to go flying without checking first.