I don't think manually performing the password hashing steps would be the solution. Should you have detected that it indeed fails, sure, you may do that at low-level to debug, but your customers will forget about it almost immediately.
Here the problem is that they are using a wrong password, either because they mistyped it, or really think they had used a different one than the one they did.
To rule out the first case, make them write the intended password in a text editor (eg. notepad), cut it to the clipboard and paste into the password field. The password is showed to the customer during the process, so that removes the cases where a wrong letter is being pressed, Caps Lock was set… (of course we are expecting that they do this at a time when nobody else is looking at their screen)
The second case is harder, as we do not want to encourage the customers to keep a text file with their passwords. Ideally, you would convince your customers to use a password manager like KeePass. Then, if the password -the same that worked before- is correctly copied from the password manager, you can be quite certain that it is indeed the right password.
Other than that, which really requires a change of mind of your customers, you may ask your customers to:
- create a new password
- change their password to that by pasting it
- try logging in with the same password they have in the clipboard
as many times as they want, in order to attempt to reproduce it.