What are the signs of a stuffed thermostat? I need to know as I have the feeling that one in my 2L RS might stuffed as I had it idling for about 5-10 minutes and the temp gauge was at the second line in the middle and it was only 15 degrees outside. I topped up the radiator about 1-2 weeks ago, I rather check the easier stuff first. I can get replacements from my local Autopro easily or grab the one from my spare 205 engine.

What you have said is the signs of a stuffed one, but it could also be a blocked radiator or the fact that it was idling presumably stationary for a long period with little air movement.  With the cost of a new thermostat being pretty cheap and it being relatively quick to change it out, it's probabally a good start.