Lithium ion battery - measure C rate How does one determine the C rate of an aged cell?

I would like to find the discharge rate just before any cliff edge of capacity loss. Can the C rate of a cell be estimated by the cells rated heat dissipation and measured internal resistance? Where I could find the current that will use up all the cells heat dissipation capacity.

I have a number of recycled laptop battery 18650 cells.

Are there any negative effects of the cells being below room temperature?
To determine the C-rate, you should charge and discharge two cycles very slowly say at 0.1 mA, between say 1.5 V to 4.3 V for your Li-ion (unless it's a very old laptop battery it should be LiCoO2), and measure how long it'll take. The C-rate would be your capacity divided by one hour.

Don't know of a way to measure it using heat or internal resistance.

Batteries generally have an adverse effect when heating up, so unless it is much below freezing, they should be ok.