Short answer. By giving security. Where the condition is negative, not doing or not giving something, you may claim the inheritance at once on posting a bond that you will not do the prohibited act, and that if you breach it you will return what you received together with its fruits and interests. You need not wait a lifetime to prove it.
What the law says
If the potestative condition imposed upon the heir is negative, or consists in not doing or not giving something, he shall comply by giving a security that he will not do or give that which has been prohibited by the testator, and that in case of contravention he will return whatever he may have received, together with its fruits and interests.
Civil Code, Article 879 — Negative Potestative Conditions (Caucion Muciana). Read the full provision →
You claim now, on a bond
Article 879 solves the problem a never-do-this condition creates. If the potestative condition imposed upon the heir is negative, or consists in not doing or not giving something, he shall comply by giving a security that he will not do or give that which has been prohibited by the testator, and that in case of contravention he will return whatever he may have received, together with its fruits and interests. So you are not made to wait, perhaps for the rest of your life, to show you never did the forbidden thing. You take the inheritance now and give a guarantee for your future compliance.
Why the law uses a security
A negative condition of this kind can only be finally proven satisfied at your death, since until then there is always more time in which you might do the prohibited act. Suspending the whole inheritance until then would keep the property idle and you empty-handed for no good reason. The security, long known as the caución muciana, breaks the deadlock: it lets you enjoy the inheritance immediately while protecting the testator's wish, because the bond stands ready to undo the gift if you ever breach the prohibition.
What the security must cover
The bond does two things. It guarantees that you will not do or give what the testator forbade, and it guarantees that, should you contravene the prohibition, you will return whatever you received, along with its fruits and interests, the income and increase the property produced while it was in your hands. So the consequence of breach is not merely losing the property going forward; it is restoring the value as though you had never taken it. The security is sized to make that restoration real, not nominal.
How to claim in practice
To take the inheritance now, you offer the required security in the settlement of the estate, in the form and amount the situation calls for, and once it is in place the property can be delivered to you. Keep in mind that accepting on these terms binds you to the prohibition for as long as the testator intended, and that a breach triggers return with fruits and interests. Read the will closely to see exactly what is forbidden and for how long, and have a lawyer arrange the security so your claim can proceed.