Let's say you're a beginner raider or a mid-tier raider and you're looking how to improve yourself in Trials so you can transition more to endgame. You will need to learn how to self-assess your performance and make improvements that won't require a raid leader.
Three things you will need to do that other endgame raiders often do:
How you died, what was the killing blow, did you get any healing, etc? Checking with others is great as well but some people sometimes recall things differently which is fine however you'll need tools or at the very least recordings to double check what happened.
Even as important is to check what other groups are doing to see if you're missing on any mechanics or tips that could help you live.
For tanks, healers and DPS, you will need to check what your uptimes on buffs, debuffs or numbers were. Anyone can tell you "gitgud" or "just do more DPS" but how can you do it?
Could debuffs have been better? Are you comfortable with your rotation during mechanic phases where you may be interrupted? Does the group positioning or strategy affect your numbers or uptimes?
These things need to be answered otherwise you may not get anywhere.
Taking a note of how you died is good but where you died is often ignored.
If you're on console or PC it's highly recommended you record your runs to see what could be improved in terms of your positioning or group positioning. Where you died or are positioned can heavily affect the group and you want to make sure you're always in the right spot at the right times.
Otherwise as we will discuss below, addons can be of immense help on PC.
Lilith's Group Manager - Death recaps are notoriously inaccurate or not very revealing ingame, so addons like Lilith's Group Manager is the most detailed way to review your deaths or other deaths in the group.
Improved Death Recap - Similar to Lilith's Group Manager, this is more for yourself than for everyone in the group and can be used to assess deaths.
RIPFilter - This addon does not give you as much information as the two above addons but simply tallies how many deaths you've had in relation to every group member which can be looked at after the end of raid.
Combat Metrics - Even after ESOLogs came out, Combat Metrics is still a great addon to look into the data of specific fights and your uptimes.
Hodor Reflexes - During a fight (provided everybody else in the group has it enabled), Hodor Reflexes can track DPS live of everybody in the group. It can be used as a check during raids to see if numbers can be improved. It also tracks Horns and Colossi.
Let's say you're in a raid and you swear by your mother's grave that Event A happened while Event B didn't happen. As mentioned above recordings can clarify what you can improve on:
Watch the video below to see how you can set these up:
Although not available in terms of USAGE to console players, all players of all platforms and servers can still look at other groups' ESOLogs to see what other people are doing, dying to and so forth.
If you are on PC, you should definitely make use of the very intricately detailed ESOLogs to self-assess your raid performance whether you are a healer, DPS or tank.