Write down three essential journeys
Pick the three things visitors most need to do. A portfolio might need project browsing, service information and contact. A catalogue might need category browsing, comparison and product detail. Use these journeys to judge a layout before its colours.
Test with your real content
Replace sample headings and short descriptions with realistic copy. Include a long title, a missing image and a small category. A design that only works with carefully selected demo content can make everyday updates unnecessarily difficult.
Check the maintenance path
Identify the editor, plugins and update process the theme expects. A screenshot cannot establish compatibility or support. Verify those details with the current publisher, and try changes on a test installation before using a theme on a live site.