To decipher a scroll, a paladin or ranger can simply use a
wand of read magic.
What is interesting is that the Use Magic Device check is to act as if the spell was on the caster's spell list. Technically, a paladin or ranger attempting to use a spell that is already on his spell list shouldn't have to make this check.
If you look at the description of activating scrolls in the SRD, this is what it says:
SRD said:
To have any chance of activating a scroll spell, the scroll user must meet the following requirements.
1. The spell must be of the correct type (arcane or divine). Arcane spellcasters (wizards, sorcerers, and bards) can only use scrolls containing arcane spells, and divine spellcasters (clerics, druids, paladins, and rangers) can only use scrolls containing divine spells. (The type of scroll a character creates is also determined by his or her class.)
2. The user must have the spell on his or her class list.
Requirements met, for a paladin or ranger using a divine scroll of a spell on his class list.
3. The user must have the requisite ability score.
Should not be a problem for a low-level scroll. Otherwise, the paladin or ranger will have to emulate the ability score using Use Magic Device.
4. If the user meets all the requirements noted above, and her caster level is at least equal to the spell's caster level, she can automatically activate the spell without a check. If she meets all three requirements but her own caster level is lower than the scroll spell's caster level, then she has to make a caster level check (DC = scroll’s caster level + 1) to cast the spell successfully.
This is the problem section. What is the caster level for someone with no caster level? Since the RAW doesn't specifically state what are the effects of having no caster level when trying to activate a scroll, I'd be willing to allow a character with no caster level to make an unmodified d20 roll (as if he had a caster level of 0) to activate a scroll provided he meets the other requirements (1 to 3), above.
Interestingly enough, it appears that the Use Magic Device check of 20 + caster level merges steps 2 and 4 into a single roll - a DC 20 roll to use the scroll as if it was on your spell list (the DC to activate a wand or staff, which only has a spell list requirement, is also 20) and the caster level check. If the character already meets the spell list requirement, it is arguable that an unmodified d20, or even a Use Magic Device check with a DC equal to the scroll's caster level +1 is sufficient to activate the scroll.
However, the above is just how I would apply the rules, based on my reading of the spirit and not just the letter. No doubt, others will tell you what is actually correct

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