Ask lots of questions - check out their website/ofsted report
Staff and pupil turnaround
Acceess to outside play/trips
Does it follow early years curriculum
What's the school ethos: happy kids, hothoused, expensive daycare for rich expats (don't put it like that!)
Class sizes
Exposure to music and language lessons
What clubs do they offer
Hours - is there wraparound care
Pta - how is it organised, what do they do (say how delighted you'd be to bake for the cake sales...)
Religion: yay, nay or 'don't want to offend ayone so we celebrate everything)
Will he be in nappies? What will he need to be able to do when he gets to school: go to the loo, eat his meal, ask for help...
They will want to see that you haven't two heads and that your child doesn't have any (obvious) additional needs (on the qt). A child who can follow simple instruction, who will sit and play, have basic language skills, and not constantly distract, whine etc (and how you deal with this if they do).
They will prefer a couple who seem to get on well and have the childs wellfare at heart (no big rows before you go in!).
Be confident, friendly... It all ok. There aren't that many schools who refuse you at interview stage (unless you do something awful!).
They will probably ask: what your child enpoys doing/fave toys and games, if he goes to any clubs or classes (not too many but enough to show socialisation), if you have any concerns about his development, are there any siblings, do both parents work/is there a nanny etc.
Have another school or two up your sleeve to compare. We were accepted to HH then were blown away by a 'wild card' school.