For finding overlapping polygons/slivers in ArcGIS 10 you want the to use the Data Review toolbar. Specifically you want to go: Data Reviewer toolbar > Feature on Feature Checks > Polygon Overlap/Gap is Sliver Check.
The Polygon Overlap/Gap is Sliver check finds overlaps and gaps that can be considered slivers
If it is an issue of aligning overlapping polygons properly then you can use the information here to help deal with that issue:
Correcting gaps and overlapping polygon (parcels) geometry
The best way is to convert this shape file to file-geodatabase and build the topology in ArcGIS for "Must Not Gaps" and "Must Not Overlap".
The following tools may be of interest depending on the specifics of the situation:
Merges the selected polygons with neighboring polygons with the largest shared border or the largest area
Integrate is used to maintain the integrity of shared feature
boundaries by making features coincident if they fall within the
specified x,y tolerance.
Moves points or vertices to coincide exactly with the vertices, edges, or end points of other features.