You don't know - and neither does the computer.
Decryption is a mathematical operation. You are given a blob of yunk. You can be fairly certain it is encrypted with a given cipher because it meets certain characteristics - either length, or hash-depth, or there is a header or footer of a given length, or some revealing information about the cipher may have been sent prior to or alongside the encrypted blob.
Then, if you're smart enough, or you have enough money, or time, or computing power, or a lot of luck, the decryption operation might occur. You can check as to whether or not you've successfully decrypted the data mathematically - e.g. does the result set fit with the function I've just run and give me the source data I started with? If so, yes, you've decrypted the data.
It's your responsibility as a researcher to decide what to do with whatever came out the other side. You may have to decrypt it again before proceeding. You may find out that what you just decrypted was nothing more sinister than ICMP_FRAGMENTATION_REQUIRED (Frak!).
The holy grail of cryptography may infact be steganographical encryption - or binary / machine language that reads as Grandmother's Cookie Recipe, but when run as an executable it actually glasses the machine. Who knows?