Imaam not having wudu invalidates entire congregations’s salaah.
It is not necessary to investigate whether the Imaam has done wudu or done properly. Dhunnu bi (a)l mu`mineena khairan.
However if one happens to know somehow that the Imaam does not have wudu. Or has done wudu by wiping over normal / cloth socks then one needs to repeat his salaah. This is because wiping over cloth socks is not considered a valid way of completing wudu according to all the classical madhhahib.