Lacey does the same thing. I totes know. Old Arabian ladies! Gee.
With Lacey, I've found that pretty much, if I'm cold enough to need a coat, she is too. That's not a great rule of thumb for most horses but it's what works the best with Lacey.
Anyway, so I usually blanket her with a lightweight blanket on nights that I know will hit the low 50's or 40's. If I'm not sure, I might leave her unblanketed but then the next day when I check her and her flank area looks a little "tucked up" or "tight" I know it got too cold for her so I'll check the weather, find out what the night before's low was and generally blanket her the next night if the low is similar.
Then, for midweight/heavy weight blankets, if I'm wearing a sweatshirt+a light coat (usually around 50*-ish if it's raining, 45*-ish if it's not precipitating) and am still slightly chilly, I generally figure it's time for a MW. For HW, if I'm wearing my heaviest coat (starting usually around 32* and lower), I know it's time for the HW.
My other rule of thumb is that before changing blankets, after bringing L in from the pasture, I'll stick my arm under the blanket. If it's toasty under there but not SUPER hot, I know she's correctly blanketed. If the air under the blanket is cold or only somewhat warm, I'll step up to the next blanket.
The other thing I look at is her fur that was under the blanket. If the blanket comes off and her fur is looking really slick, I know that she was either too hot or perfect (that's why I use this method+sticking my arm under the blanket together). If her fur is at all fluffed under the blanket, she got/is too cold and needs a heavier blanket.
It's a bit unorthodox but it works for us. I've never had L sweat under a blanket or get too cold (except for once when she was only wearing a sheet and it unexpectedly dropped down to 29* overnight AND snowed, IN APRIL) so I'd say it works pretty well.
Basically, Lacey has her "tells" and I'm sure Thelma does too. Once you learn what Thelma's are, it'll be the easiest thing in the world. :)