Is the caster of Zone of Truth aware of the save result?

Question

https://www.d20pfsrd.com/magic/all-spells/z/zone-of-truth/

If I cast this spell, will I be able to tell whether the target saved or not?

The spell level and therefore the DC will be too low so my intention it to use it for negotiations where the target and myself both decide to fail it to make the negotiations more reliable.

In case I can't be aware of the result (which makes the spell completely useless since everyone can save it), is there some other spell that can be used to reliably prevent lies?

Answer 1

For the first question

As pointed out in the comments, the first part of your question is a duplicate of this question.

For the Second question

There are a few options I can think of to achieve what you are looking for.

Touch of Truth Telling

Touch of Truthtelling acts as a Zone of Truth but only has a single target and the spell has a very visible reaction.

The target momentarily takes on the semblance of a being of perfect order, like an archon or inevitable, so that all who can see the target know it is affected by the spell.

Both parties in a negotiation would need to have the spell cast on them, but it is a first-level spell so the cost is minimal.

Mark of Justice

Mark of Justice is a spell which allows the caster to designate an undesireable action, and if that action is taken a curse comes into effect. If the triggering action was "lying to the caster during this negotiation" or something similar it could reveal a lie. It only works on someone willing to take the 10 minutes to do it. But it would give more dire consequences to a betrayal.

Fairness

Specifically for trade negotiations, Fairness forces those involved to trade fairly. Since its target is creatures/level instead of an area, the caster would know if their target saved against the spell.