Methods and systems for cementing casing in a wellbore are described. The methods include introducing a modified biopolymer into an inner diameter of the casing, reverse-circulating a cement composition in the wellbore, permitting the modified biopolymer to contact the cement composition, and terminating the reverse-circulation upon detection of an increase in pressure in pumping the cement composition. The modified biopolymer includes a reaction product of a biopolymer and an organic carbonate, and the cement composition has a pH greater than 11.