Can I still sue the seller if I only found the problem by accident, not because I checked?

Yes, generally — how you happened to find a problem doesn't affect your underlying legal right to raise it. What matters for an indemnity claim is whether the negotiated survival period for the relevant representation is still open at the point you actually discover the issue, not whether you found it through diligent checking or by pure chance.

Similarly, Ontario's general limitation law is built around a discoverability principle that generally starts the clock when a claim was discovered, or reasonably ought to have been — not on how the discovery happened. In practice, this means an accidental discovery is treated the same as a deliberate one for timing purposes, though it can matter to the strength of your evidence if you now need to show exactly when and how you learned of the problem. Document how and when you found it as soon as possible, since that record can matter later even though the manner of discovery itself doesn't weaken your legal position.