How will I know how s/he's doing academically?
The only way is to ask him/her. UK universities are not permitted to release information about academic progress, attendance or anything else to any third party without that student's express consent. This includes student parents.
Examination results are issued regularly to every student on a taught course - usually after the end of each semester.
If they inform you that they are facing academic problems, try to find out why. University work is very challenging and even A+ students may receive their first poor grades at this level.
University of Sheffield regulations allow students to re-sit failed examinations and even the most hard-working students sometimes have to do this.
Every student is allocated a personal tutor, who will be available to discuss academic or personal concerns on a one-to-one basis.