To get a good, reliable estimate on User Stories (more general: software requirements), you can use the method of Planning Poker: The game master (for Scrum projects: the Scrum Master) presents a requirement (in "Scrum"-based projects: a User Story); the participants of the Planning Poker write their effort estimate (for example in Story Points in "Scrum"-based projects, else: in hours or days) on a piece of paper and place this piece of paper upside down in front of them on the table. When all participants have written their estimate on their piece of paper, the game-master calls for turning over the estimates/pieces of paper.
If there are (strong) differences in the estimates, the players with the lowest and highest estimate are asked to justify their estimate. This explanation creates a better understanding of the requirements or the challenges in their implementation for all those involved. The estimation round is then repeated. The process is continued until the estimates are so close together that a consensus within the group can be assumed. This usually takes no more than three (3) rounds.
When the members of a development team sit together, a physical card game can be used, too. Of course, there is also a "digital" version of Planning Poker, which is especially suitable for distributed teams. Dozens of websites offer the opportunity to use Planning Poker free of charge, with participants each providing their estimate by means of a digital playing card. Of the dozens of free providers, the following should be mentioned: www.planningpoker.com