Well, I imagine you could. First, I presume you're talking about a barrel that's clean and not fouled with gunpowder residue and carbon deposits. If it's black with gunk nothing but the gunk will be visible.
So, starting with a clean barrel, shine a flashlight through it. Is it shiny or dull? Do the lands and grooves stand out, or are they blurred? Are there visible lumps of some material stuck in the lands? Dull looking deposits or obliteration of the lands and grooves is a pretty good sign of leading.
If you think you have leading, don't despair. There are ways to get rid of it and it isn't all that hard. First, many of the shooting supply houses sell lead removal tools which consist essentially of a brass screen that wraps around a slug (also brass) and which you push down the barrel. That manually scrapes the deposits off. There are also all kinds of chemical solutions to the problem. I clean my barrels by running patches that are saturated with J & B Bore compound (available from Brownell's) through it, then brushing it thoroughly with a solvent-soaked bronze brush and, finally running solvent soaked patches through it. That will get the lead out, although it may take some elbow grease to do a complete job. There are lead removal cloths which you can cut into strips and run through your barrel and these work, too.
My last thought is that leading is not all that common a phenomenon. Most modern lead bullets are sufficiently hard that they don't leave deposits, as a general rule. The one leading problem that I'm aware of is if one uses a fairly soft lead bullet on top of a magnum charge. The heat and velocity generated by the magnum round may cause lead from a soft bullet to scrape off as it goes down the barrel.