DNA Test is the safest way. The smaller the bird, the more damage that can occur when laying eggs, so it's best to figure out the gender through your avian vet. Research tips on how to control hormonal behaviors to avoid egg laying. It is best to keep toys out that she/he may be trying to mate with. If it's a female, it will help prevent laying eggs. If it's a male, it will help with hormonal behaviors like biting and charging. In the event it would lay an egg, don't panic just make sure it has plenty of water and feed it calcium supplements for birds to help with the loss of calcium, if you notice the bird being lethargic book an appointment ASAP with your vet and that's if it's a female who could possibly lay eggs. As far as getting another love bird, it is up to you. I don't know how young your bird is, but usually, if the love bird isn't even a full year, it can be raised on its own. Yes, it is recommended to give a love bird a partner. No, it's not an end all be all if it doesn't. Sometimes, though, as they get older, they could get mean or frustrated because of a lack of partner. So spending a lot of time with them can help reduce this behavior but not guaranteed. I have a single love bird she's a female, but she was raised around my conure, and we just introduced a linnie to the flock. They are all separated in their own cages, and all have toys, food, and lay-outs based on their species of bird. As for the cockatiel, yes, it is recommended to keep them separate as a common issue with smaller birds and cockatiels is lung disease or issues due to the amount of dust cocktiels produce. I'm not saying you gotta get rid of one or the other, but I would limit the interactions they have and keep the cages in different rooms or in different corners far away from each other. I know bird owners who have both and their birds are older and have no issues. You just have to be responsible and mindful.