To identify a frame firstly see if a magnet sticks, if it does it's steel, if not it's carbon, aluminium alloy or titanium alloy.
If not steel look down the seat tube if it's metallic inside it could be aliminium or titanium if black and plastic looking, carbon. Tap the frame with a screw driver, aluminium and titanium will have a definite metallic 'tink' and carbon a hollow 'thunk'.
To tell the difference between alloys put a small scratch in the seat tube (both have good anti-corrosion properties) if the exposed metal is dark and dull silver it's possibly titanium, if light silver it's aluminium. Titanium frames are expensive and rare so not likely to come across one you don't know the pedigree of.
For steel there is a significant difference between the two main types of steel, Chromoly and Hi-Ten steel. Chromoly is a stronger and lighter (high carbon) steel and is generally found on mid to high end bikes. If a bike is reasonably modern and chromoly it is likely to have smaller diameter tubes and possibly seat post (although this isn't a good indicator as new steel frames are being produced to fit dropper posts). A chromoly bike may have a sticker on the seat tube from the tube manufacturer be it Reynolds, Tange or Columbus. Tube shape may also vary on higher end frames (my chromoly HT has a triangle top tube and oval down tube). A Hi-Ten frame will mirror the fashion of when the bike was made.
Few Chromoly frames come as built up bikes so generally if you don't know your riding Chromoly, a magnet sticks to the bike and it's heavy you're riding a Hi-Ten steel bike. The scrap value of the frame is probably worth more than the frame.