of Lines and Rods

Is there some clever way to identify line weights?

I seem to have amassed a selection of lines, but I have no idea what
weight most of them are. This is because my primary rod for several
years was a handmade rod that was "in the 4 or 5 wt range", so I ended
up trying out and accumulating a selection of 4 and 5 wt DT lines,
some sink tips, all in different brands.

Now I own a bonafide 4wt (8 ft) and a 5wt (9 ft), so rather than buy
new lines, I'd like to figure out what goes with what. I've tried
casting them with the different rods, but unfortunately my casting is
so mediocre, I really can't tell the difference. The only one that
casts REALLY well is the 5 wt that came with the 5wt rod.

But my instincts tell me that there should be a significant difference
between a 4 wt 8-ft and a 5 wt 9-ft, so I think the problem is
psychological....since I'm not certain the line fits the rod, I don't
have the confidence casting. So I'd really like to figure out the line
weight, rather than toss 5 or 6 perfectly good lines and buy new ones.

--riverman