Your best bet is to post a picture, preferably drive side. Then post close ups of the lugwork: head tube and seat cluster where the four tubes come together. Close ups of the drivetrain are useful, too: crankset, rear derailleur and dropout, etc. A photo of the brake bridge at the rear is often helpful.

Also, post any serial numbers that are on the bike. They are often on the bottom bracket or seat tube near the bottom bracket. Sometimes, they are on the rear dropouts or head tube.