How can you tell if an oil painting is dry?
Before providing a way to test the oil painting is dry, it's useful to know how the drying process works and why it is important.

How does oil paint dry (cure)?

Oil Paint doesn’t really dry, rather it cures. The pigment is dispersed in oil (typically linseed) and generally contains a solvent (methylated/white spirit). The solvent evaporates away leaving the oil and the pigment.

Then over a period of time the oil &amp; pigment slowly become less malleable through a process of oxidisation. The oil hasn’t evaporated away (dried) it has become hard (cured).

What can cause differences in the drying time?

As you have mentioned in your question there are several things that can affect the curing time of the paint.


Application thickness of the paint
Pigment colour:


Earth colours require less oil (ochre etc.), so cure quicker

Primary colours require more oil (crimson etc.), so cure slower


Brand/type of paint
Environmental factors such as temperature, humidity, airflow &amp; sunlight


Most paint suppliers offer good advice on how long their paints take to dry. But some general guidelines are that most paints will be touch dry after 10-14 days and should be fully cured within 6 months. However, for those pieces where the artist has used a liberal impasto technique, then upto 2 years is possible!

Why wait?

If the artist intends to varnish, and there are several reasons for this:


Protection
Bring richness &amp; vibrancy to colour
Ease of cleaning


Doing so before the paint has cured could lead to cracking and also it might not allow the paint to fully cure, and this can lead to weakness between layers. And, therefore, flaking.

When framing it is less important that the painting is fully cured. But there are risks as the piece could be knocked during the process and cause indenting, or squashing, of the paint. Also, it can slow down the curing process.

So to answer the actual question: 

How do you know that the oil painting is dry?

There is a very simple test.

Take a lint-free rag and dip it into white spirit. Pick an inconspicuous area and gently rub. If any colour shows on the rag, it requires further drying time before varnishing or framing.

But I wouldn’t suggest doing this on your newly acquired Gerhard Richter.