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How do I know my paper wallet key is valid?

These are two questions, and separating them matters

"Is my key valid?" hides two different things. One is whether it was copied correctly — that no character was misread off the paper. The other is whether it holds funds.

The first is answered offline, exposing nothing. The second needs to look at the chain, but that is done with the address, which is not secret. Conflating them is what leads people to paste a private key into a website to answer a question that never needed it.

The key checks itself: WIF carries a checksum

The format almost every paper wallet key is written in — the one starting with 5, K or L — is WIF, encoded with a scheme that appends four bytes computed over the rest.

So if you misread one character while transcribing, the arithmetic does not line up and the error is caught. It is the same idea as a check digit on a bank account number: it does not prove the key is yours or that it holds anything, it proves it is intact and correctly copied.

Any offline WIF validator answers this in a second. No importing, no connection, nobody told which key you hold.

A zero balance does not mean the key is bad

This is the trap that makes people conclude they lost everything. One private key can correspond to more than one valid address, depending on the format the generating software used. Check one, see zero, and the funds may sit at another address derived from the same key.

This is not speculation. There is a standard written specifically to close that gap: BIP-178 describes itself as "An extension to the Wallet Import Format (WIF) to specify what kind of bitcoin address the private key corresponds to" — BIP-178. That it had to be written at all tells you the rest: plain WIF does not say which address it belongs to.

So before concluding there is nothing there, check whether the address printed on the paper is the one you are querying — and if not, try the other formats that key can produce.

What not to do to check it

Do not type it into a website. Even an honest site means the key left your machine and is outside your control. Validating the format needs no connection at all, so a service asking for your key to "verify" it is asking for something it does not need.

Do not send it to anyone. Asking about this in a forum reliably produces private messages offering help. There is no legitimate reason for a third party to need your private key — not to validate it, not to check a balance, not to recover anything.

Do not import it just to look. Importing loads the key onto a computer and that decision cannot be undone. Checking a balance does not require it.

The order that makes sense

First confirm it was copied correctly, offline. Then look up the printed address on the chain, which exposes no funds. If a balance appears and you want to move it, the right operation for a paper wallet is almost always to sweep — send everything to a fresh address you control — rather than import, because you do not know how many copies of that paper exist.

And if nothing appears, check the address format before writing it off. That is where most of these end.