回複：最近發現前房主動過地基，還可以追訴前房主嗎？

If the seller'sdisclosure didn't include foundation work, it still will not mean he is at fault, as the work might be done by previous owners.  Anyway a couple of ways you can try:

1) contact City permitting department asking for permit record.  Foundation work requires City permit, so you may see the record past repair/renovation work done to this property. 

2) contact a few foundation companies of your area, asking them if/when they have done work to this house.  They may have kept record.  Try to ask tactfully. 

First of all, get a professional to do a check and provide you with a quote to see if it IS a big deal.