first you need to find out if the algorithm is veracrypt or truecrypt or diskcryptor. These are 3 different algorithms.
I would suggest to find out what you see (in case you need to input the password at boot time) when trying to unlock/decrypt the disk and maybe google the phrase/prompt/description. Most of these have some kind of bootloader that can be distinguished from each other if you see some password prompt/field with text @ boot time.
If you can't really boot it and do not have any idea what software was used, it could be much more difficult to find out the algorithm and the data needed (and of course some full-disk-encryption algorithms make the data as random looking as possible and without any hint of which algorithm/software was used)... therefore I would say without the bootloader prompt text or an installed software on another computer where the disk was used/decrypted in the past (e.g. if only truecrypt was installed that's already a hint) you can't know for sure.
I would suggest to find out what you see (in case you need to input the password at boot time) when trying to unlock/decrypt the disk and maybe google the phrase/prompt/description. Most of these have some kind of bootloader that can be distinguished from each other if you see some password prompt/field with text @ boot time.
If you can't really boot it and do not have any idea what software was used, it could be much more difficult to find out the algorithm and the data needed (and of course some full-disk-encryption algorithms make the data as random looking as possible and without any hint of which algorithm/software was used)... therefore I would say without the bootloader prompt text or an installed software on another computer where the disk was used/decrypted in the past (e.g. if only truecrypt was installed that's already a hint) you can't know for sure.