What type of proof to show there was demand in estafa?

No specific type of proof is required to show that there was demand. Demand need not even be formal; it may be verbal. The specific word "demand" need not even be used to show that it has indeed been made upon the person charged, since even a mere query as to the whereabouts of the money [in this case, property], would be tantamount to a demand. As expounded in Asejo v. People:

With regard to the necessity of demand, the Supreme Court agrees with the CA that demand under this kind of estafa need not be formal or written. The appellate court observed that the law is silent with regard to the form of demand in estafa under Art. 315 1(b), thus:

When the law does not qualify, the Supreme Court should not qualify. Should a written demand be necessary, the law would have stated so. Otherwise, the word "demand" should be interpreted in its general meaning as to include both written and oral demand. Thus, the failure of the prosecution to present a written demand as evidence is not fatal.

In Tubb v. People, where the complainant merely verbally inquired about the money entrusted to the accused, the Supreme Court held that the query was tantamount to a demand, thus:

x x x [T]he law does not require a demand as a condition precedent to the existence of the crime of embezzlement. It so happens only that failure to account, upon demand for funds or property held in trust, is circumstantial evidence of misappropriation. The same way, however, be established by other proof, such as that introduced in the case at bar.

In view of the foregoing and based on the records, the prosecution was able to prove the existence of all the elements of the crime. Private complainant gave petitioner the pieces of jewelry in trust, or on commission basis, as shown in the receipt dated May 2, 1991 with an obligation to sell or return the same within sixty (60) days, if unsold. There was misappropriation when petitioner failed to remit the proceeds of those pieces of jewelry sold, or if no sale took place, failed to return the same pieces of jewelry within or after the agreed period despite demand from the private complainant, to the prejudice of the latter. (G.R. No. 180016; April 29, 2014)