How can I quickly estimate my level in go by playing against a computer/tablet?
There are several rating systems, for most players their KGS rating and EGF rating will be different, and it can be different from national rating systems (e.g. it is often assumed that French and British ratings for the same player are 2 stones different). So, in a sense, your question cannot be answered unless you specify the rating system you want to be measured against.

The benchmark could be a KGS rating, because KGS is so widely used. The best way to know your rating is to play on KGS with the real players, and KGS will answer your question.

It might be more psychologically comfortable to play against robots, but, from a personal experience, I would not recommend it, they do not help you to improve at the rate that the games against humans will. Also, they have flaws that you might identify and this will allow you to win the games against these robots, but this won't translate in your improved ability to play against humans.