As a rule, only permanent items of pay are pensionable. This will include any allowances your employer tells you are pensionable, but will not include some payments, like overtime.
Bonus payments do not normally count as pensionable earnings, but if you receive pensionable bonus payments, your employer (and you, if you choose to contribute) will pay contributions on them. You may also have some non-cash pensionable earnings. For example, some people may receive a 2% uniform allowance, and others may have an allowance for accommodation. In these circumstances, your employer (and you, if you choose to contribute) will also pay contributions based on the equivalent cash value of these non-cash pensionable earnings.
Your payslip should show the level of your pensionable earnings.