How would I know if my 914 is overheating?, Here we go... Options

[1972 914 2.0]

This may sound like an odd and silly question but here it goes:
How do I know if my 914 is overheating?
I just rebuilt the motor over the summer: 2.0L, Euro pistons/cylinders, mild WebCam, new bearings throughout, rebuilt heads, Weber 40s, running 20W-50 Valvoline conventional.
Its a 72 914 with no gauge or anything to indicate temperature.

The reason I ask this question is I've been having some rough running issues lately (past week or two). Before that the car was running an absolute treat.
Symptoms: rough running, hesitant, irregular idle, and it wont rev smoothly.
I think its ignition (just ordered new distributor, coil, and pertronix yesterday) or fuel.

The reason I think its overheating is it seems to run fine for the first 10ish minutes I drive it runs fine and even warms up fine.
And then its like a switch is flicked: I put up to a stop sign or change gear and it starts running like shite.
Either way I'm changing the ignition components but if its over heating then I don't want to drive it until figure that out.

So heres a conundrum for all you teener-heads to ponder while you sneak off to the garage to avoid your relatives on this beautiful holiday. (IMG:style_emoticons/default/type.gif) (IMG:style_emoticons/default/hide.gif)

Oh and Happy Thanksgiving!