When is it reasonable to suspect blood clot on a leg? noticed small segment of vein on leg that is little darker than usual. i do sit for long time...

Unlikely: Clots would cause pain and swelling of the leg It is unusual to have vein on leg as a cause of clot

DVT: The clots you need to worry about are deep in leg not in the superficial veins you can see, Deep vein thrombosis -- a blood clot in a deep vein, often in your leg -- can look like many other health problems. And half the time, DVT causes no symptoms http://www.webmd.com/dvt/deep-vein-thrombosis-dvt-symptoms-diagnosis#1

Deep vein clots: Clots in the deep vein system are the ones of concern. May be found from inactivity or restriction of venous flow from constriction or compression or dehydration or related to other illness. Need to watch for pain, swelling, discomfort when walking or moving foot/ankle. If concerned need venous dopplers and physical exam