Contract signing and signature forgery

When signing a contract, how can we make sure the other party's signature is real? what if that person scribbles some random signature? And if disputes arises he can claim that it was a forgery.

What does the law has to say about these kind of situations? what preventative measures can you take?

This is what notarization is for. If you are concerned about forgery - have the document signed in front of a notary and notarized. The notary will insist that the person provide some form of identification and attest that the person actually signed the document.

Where are you located? Who are you doing business with? These are very important details you are omitting.
Without further details there is only one answer: if you are that worried, do not do business with this person/company.

In my career I have learned not to do business with people I would not do a handshake agreement with and sleep well after.