Where the water shows up
Where you find the water is the best clue to where it gets in. Walk the basement during or right after the wet spell and look for:
A line of water at the floor edge
Water seeping in where the wall meets the floor.
A wet stripe down the wall
Water coming through a crack.
Water under a window
A window well that fills up and spills in.
Puddles in the middle of the floor
Water pushing up through floor cracks or a floor drain.
Damp blocks with a white crust
Water moving through block and leaving minerals behind.
Wet spots high on one wall
Often a gutter overflowing or a downspout dumping right there.
A sweaty floor and walls in summer
Humid air, not a leak.
A sump pit that overflows
The pump cannot keep up or has failed.