How to evaluate student team members who did minimal work on a team assignment?

Evaluating your fellow students should be a supervised exercise of constructive criticism, e.g., after a presentation given by them. I hope this is what the lecturer will do.

Anyway, if you are assigned this you need to do it. I would give the bare facts for the students that didn't really involve themselves. I.e., I would state exactly what each team member contributed to the project. And if they only suggested a color for a button then I would state that. I would then proceed with providing an honest evaluation of the student that did all the work with you and for the others just say that they could have involved themselves more in the project.

I know that this feels a bit like being a tattletale, but it is actually them that behaved unethically by not working with you on the assignment. The only thing you should have done differently is having a frank discussion regarding their involvement (with them and with the lecturer) at an early stage. Resolving issues like this during team work is actually something you need to learn during your studies. And it appears like you haven't. Thus, you should add this as self-criticism to the evaluation and ask the lecturer to teach you methods of handling such issues.

Since it appears that you did actively try to engage the other members of your group in the project, I will answer based on that assumption. If you did not actively try to involve them, well that is a different problem to address.

I have had a number of such encounters throughout my time in school, and my response was to be as objective and honest as I could be without looking like I was trying to put them down. Just state exactly what they did to help the project, nothing more, nothing less. If they didn't do anything, say that. Their grade is not your responsibility, so if you are asked to review their work, don't make it your responsibility to make them look good. Simply state exactly how much work by each person, and leave it to the professor/instructor to assign the grade.

Be careful to not let personal feelings get involved, and to grade everyone by the same standard. You could even create a rubric or break down involvement by sections of the project.

In my opinion, trying to make it look like they did more in the project than they actually did, so that they get a better grade is actually doing a greater disservice to them, than giving an impartial review that shows they didn't really do much. The reason for this is that it can reinforce their theory that they can get by without really doing much work, and relying on other people to pick up their slack, and if they continue on like that, it will most likely come back to bite them later. Better that they do poorly in a course now and learn they can't, than learning it later by possibly getting fired from a job.

Note: Be sure to consider whether or not you and the other active member in your group may have shut out the other members from participating, thus relegating them to a position where they weren't able to do much to help with the project.

Internal peer reviews can provide information that an instructor simply can't glean from a written exam and oral presentations.

I am currently teaching a course where the students have to do an end-of-semester presentation on a project they've been working on for the last six weeks or so. Each team has six members, and the time schedule we have available means I have just about five minutes per group to ask questions. That doesn't give me a lot of time to investigate group dynamics and decide who was productive and who wasn't on a meaningful level.

I therefore rely on a peer evaluation to help understand what went on in the group. Did everyone contribute their fair share, or did a few people "freeload" of the rest of the group? Did people work together well, or was there a lot of friction inside the group? Sometimes I can tell this from the presentation, but a lot of times, I can't.

However, this only works if everybody participates honestly. You can choose to decline and say nothing about what the other group members have done or not done, but then you also have to be willing to live with the consequence that those students will get the same grade as you have earned without doing anything to actually earn the grade.