How a tools knows it's successfully decrypted a file?

Hi, folks,

A newbie question (I couldn't find the right Googlism to hit a result,
nor did the FAQ shine a light).

I've used a couple of tools to encrypt and decrypt files for me based
on a password of my choice. When I try decrypting an encrypted file
with the wrong password, tools generally respectfully tell me that I've
made a tit of myself, and ask me to use the right password.

I hadn't thought much about this till the other day, using a new(ish)
tool to decrypt. I entered the wrong password (unknowingly, of course)
and it told me that the file was decrypted successfully; but a look at
the contents told me that this was not so. I think it just re-encrypted
the file using the, "New," password.

My question is: how does a tool know that it's successfully decrypted a
file?

I can think of a way of doing it, but it seems a little ... insecure:
the tool adds the password to the plaintext before encryption and then
encrypts it along with everything else. Then, after decryption, the
tool checks that the first X bytes match the password, and if so:
success.

Can anyone answer my question above?

Thanks for your time,

..ed

.

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