ASA (like ERC-20 on Ethereum) don’t have inherently a value.
What gives them value is if people are interested in buying/selling them.
Usually this happens on exchanges.
Note that there may be multiple exchanges and each exchange may have different value for the same ASA.
If the ASA is listed on Tinyman for example and you are interested in the value of the ASA on Tinyman, see Is it possible to read the price of an asset on Tinyman from a stateful smart contract? Kind of like a on-chain oracle? - #2 by Ben
Some projects may also be listed on websites such as Coinmarketcap or Messari for example. These websites aggregate the value of the tokens over multiple exchanges.