Using Layout, trace width can be checked using the Fabrication Check of Verify Design, but it really only checks a minimum width.
Trace widths can be manually checked using the Find Tool. Use the drop down box to select Line Width, select the Line Type of Trace, then select the width(s) you want to examine. This is the most thorough check of all existing line widths, but it requires manual interaction.
Using Router, min/max trace width checking can be done using Design Verification. It's still not as thorough as using the Find tool, but it might be enough depending on what you're trying to check.