A skill that's useful when someone comes to you for validation popover available is Active Listening. Active Listening is a set of skills which show you are paying attention and make the speaker feel heard. It doesn't mean being a yes-man. It's about making it clear you understand what they're saying. 
When you are trying to actively listen, you avoid interrupting, judging, or giving unsolicited advice. Your goal is to focus as much as you can on showing that you're listening, showing empathy, and showing that you understand (or are at least trying to).
Showing You're Listening
Three simple things can really help show someone that you're paying attention and trying to understand. Encouragers, asking for clarity, and if you're both people who use body language to communicate, then body language as well.
Encouragers are little words, phrases, comments, and noises, which encourage someone to keep speaking. Examples of encouragers are "uh-huh," "oh," "I see," "go on," "mmmm," "really," and "and then?" If you overuse these, you'll sound phony and shallow. Use reactions when they feel appropriate. For instance, if someone says something they think is surprising, that is a natural time to express surprise with an "oh wow" or "really." Use encouraging statements if you think the speaker is starting to feel like they've been talking for too long. If you genuinely do want to be listening, then expressing that by saying "keep going" or maybe a series of "yeah"s can be really helpful in the conversation.
Asking for clarity is what it sounds like. Choose a detail of what the other person is saying and ask for clarity. It is important that it's a factual detail, such as what order something happened in, to avoid sounding like you're judging. It's good if you pick something you genuinely aren't sure you understand, but even if you understood, asking a clarifying question anyway is still a way to demonstrate that you are paying close attention and want to understand. An example of a good clarifying question could be "So wait, was it your glass or his that got knocked off the table?" or "Did it start raining before or after you got in the car?"
Finally, if you're both people who use body language, then using body language more intentionally can really show that you're listening. Make eye contact, face forward, lean in, nod your head, keep your body relaxed, and avoid doing other things that make you look distracted like checking your phone or walking out of the room. 
Sometimes, people have trouble identifying and accepting their emotions. They might not be sure what emotion they're feeling, or, they might know what emotion it is but aren't sure if they're justified in feeling it. Naming emotions is a strong way to show that you're listening and understanding. It can also help someone sort out how they feel, or validate them.
As you listen, try to work out what emotion the speaker is feeling, or how they feel about the topic. If you're able to use tone of voice and body language cues, then those can be immensely helpful. Otherwise, try thinking about how they're describing things. Are they swearing between words? Are they expressing that they liked something, or that they dislike it? Why?
When you think you've figured out how they feel, find a point in the conversation to say something like "Wow, that sounds so frustrating," "That would make me feel so scared," or even "You seem happy." This may sound shallow, but it can actually feel really good when someone names how you feel.
Don't tell them not to worry or be stressed as that comes off as dismissive. Just acknowledge that they feel it.
Finally, it's important to show the other person that you actually understand what they're saying. The main way to do this is by paraphrasing. This is akin to meta-conversing popover available. This both shows that you understand, and also confirms to you that you actually do understand.
To paraphrase, find a point in the conversation to repeat back what you believe the other person has been saying, but in your own words. Here's a fake conversation as an example:
Ollie: "So this guy tells me to take my 'conspiracy theories' somewhere 'less academic' and then chuckles with his friend."
Chyrell: "Wow, so you're saying he insulted your research and then laughed at you?"
Ollie: "Yeah, exactly! And so then..."
As you can see, Chyrell has said nothing that Ollie hadn't already said. She just echoed his sentence in her own words. Because they were not the same exact words Ollie used, it demonstrates that she genuinely understood what he said. Because Ollie feels understood, he feels better and also feels safe sharing more. [1-3]
Remember Your Boundaries!
It can sometimes be easy to get caught spending lots of time and emotional energy actively listening to someone venting on and on to the point that they might feel great, but you're exhausted. Please remember that these are just skills to employ when you think it's helpful and honestly want to be listening and helping someone process like this. It's important to be mindful popover available of how you're doing, and to politely put a stop to conversation if it's going on past your ability to keep going. It's perfectly fine to say "Listen, I'm actually feeling really tired." and "Could we actually talk about something else? I don't think I'm up to this right now." Just because active listening involves encouraging them to keep going, does not mean you're obligated to keep encouraging them to keep going when you don't want them to.
 Lee Scheingold. (2008, August 11). Active Listening. RelayHealth.
 Jessica Ortiz. (2016, Spring). The Art of Active Listening [pamphlet]. Hampshire College.
 Mary Gander. (2016, Spring). Active Listening Exercise [pamphlet]. Winona State University.