Everybody says to inspect your furnace filters, but they don't say what
to look for. On the TV news they always say it, then show a filter that
is completely clogged. Obviously they should be replaced before that.
So what should I look for to decide if I need to replace them. I have
been replacing them every 3 months, but when I do I can barely tell
the difference between the old ones and the new ones.

Bill