It's a .., no, hang on, it's a ..., er, no, I was right the first time, it's a ...

And then some obliging knowall arrives and turns your Arctic Tern (or whatever) back into a Common Tern (or whatever), which is what you were secretly afraid it was in the first place, erm, no, the second place....
My philosophy about identifying wildlife is "It's not where you start, it's where you finish that matters." The corollary of this is that you should never be afraid to CALL, even if you then have to change your mind - or in my case, have my mind changed for me.
One of the most important lessons you can learn is that different people depend on (plump for) different characteristics when trying to id or separate similar species. In the case of Arctic vs Common Tern, for instance, you might be looking for the bill colour (assuming it's an adult in the breeding season). Personally, I can't get that blood-red versus orange-red distinction, and black tips get mixed up with the floaters in my rheumy eyes. Or you might be looking at the pattern of the remiges to see if there is a translucent window (That's the one I usually try for). Or you might not be looking at plumage at all, but instead judging the bird on behaviour or movement: depth of wingbeat, how it feeds, buoyancy of flight. Or you might be one of those lucky people who can separate them on call. Or if it's resting, leg length, extension of primaries, well, you get the idea......
What you should do is this: try out all the various criteria that are available, find the one that suits you best, and use it to make your first call, then use others to confirm or contradict your first call. If there are others with you, listen to what they say, but always make up your own mind.
Put another way: you cannot claim to have seen the bird on the basis of someone else telling you that's what it is, but only after you have satisfied yourself. Put yet another way, you can identify the bird only when you are sure you could identify another of the same species at another time and in another place. I have a (mercifully short) list of birds that I am told I have seen, but which I have not really seen. An example: I went to Minsmere to join some friends who were watching a Semipalmated Sandpiper. It was on a distant mudflat with other peepy waders. After searching for a while, I asked "Which one is it?" and got the reply "It's the one that's limping." Thank you, but no thank you.
Only yesterday, my friend Peter called a Hobby. It was high and soaring. I got on to it with my current opera glasses (My god, Zeiss take a long time to repair binoculars) and simply couldn't make a Hobby of it. Peter, with his amazing Swarowskis, had seen the face pattern even at that distance. I continued to follow the bird still not able to call Hobby. It wasn't that I doubted my friend's identification, it was just that I couldn't call it a Hobby: it still wasn't MY Hobby. . Thankfully, it "jinked" suddenly and offered me a perfect Hobby profile, so Peter and I could part company still good friends.

Grandpa, I think birding is stupid.
I'm not a birder, dear, I'm a birdwatcher.
Oh? What's the difference?
About 30 years, sweetheart.
Grandpa, I think it's time for your afternoon nap.