How do you know how much a certain painting is worth?

The art market is a bit insane, but the general rule is a work of art is worth whatever someone is convinced to pay for it. Take Jackson Pollack, for example. Before Peggy Guggenheim discovered him and put her influence behind him as a major artist, you could have picked up one of his works for next to nothing. Basquiat is another good example--if he hadn't been made fashionable by certain dealers, no one would have heard of him or bought his works. One of the major artists in the center of this financial firestorm is Cy Twombly. Dominique de Menil, who had otherwise excellent taste in art, built an entire museum in his honor, although many non-critics (and I'll put myself in that category) don't see what the fuss is all about.

Age, size, style, etc. generally have little to do with the worth of a painting. Vermeers are tiny, yet they'll pull down more bucks than a mammoth scene by Church, for example. Some of the most expensive paintings in the world were created in the last 100 years (think Van Gogh and Pollack, again), but I can run down the road to an antique dealer and purchase an original early 19th c oil for a fraction of what a Van Gogh is worth. And since tastes fluctuate over the decades, what is worth a pittance today could be worth a fortune tomorrow, and vice versa.

In sum, a painting made in the early 1800s wouldn't necessarily be worth more than a few hundred dollars, although it could also be worth thousands.

a painting is only worth what someone is willing to pay. if someone with a "big" enough name starts saying your art is great and that is the new thing, your art would sell for much more then you would think. its pretty messed up, the best thing to do is make only a few of each peice and make a big deal out of the fact that there are not that many out there. you really need to know an art dealer to get good money for your art.

Depends on whether you mean worth in terms of aesthetics or worth in terms of money - the two are frequently mutually exclusive. Monetary worth depends on many factors, rarity being but one. Dying is a guaranteed way of improving the monetary worth of your work, but not to be recommended if you want to take advantage of the extra cash. Notoriety is also useful, especially if you are also deceased. If you want to work out how to put a price on your work, then the formula given above is a good starting point.

You need to calculate how much did you spend on materials and supplies : colors, brushes, mediums, canvas, electricity and etc., how many hours you worked on painting , then add 50% of all cost - here you have a price of your art piece.

I guess by who the artist is, the size of the painting, what medium was used, how long did it take...and so on.

There are quite a lot of factors.

I don' think you will find a good answer here for that but the best thing to do is ask an expert near you about it.

By evaluation of age, artist, period and style, as well as talent.

um i dont kno but take it to a action and find out