Check any radical bookshops you know of and look for anarchist papers or leaflets there. If the local group is fairly healthy they should be publishing something regularly. Check national anarchist papers for local contact addresses as well. Check a few times as smaller groups may only be publishing every few months and check all the bookshops you know of as some bookshops, particular those controlled by authoritarian socialists will refuse to stock anarchist material. If you find a bookshop that stocks a lot of anarchist stuff ask at the counter if they know how you can get in contact with local anarchists, it may well be that there is an anarchist working there.
Keep an eye out for old posters or stickers that may have a venue or an address on them. If it's a venue but you have missed the meeting try checking it at the same time on the same day of the week as often a group uses the same time and place for both public and private meetings.
Your best chance is probably demonstrations, particularly big ones that have been called by several groups (a lot of anarchists don't turn up to Leninist front demos (i.e. demos called by just one group)). Depending on how organised they are the anarchists may have a banner, leaflets or be selling papers. They may also be marching with a general community or campaign group. Some groups won't have anarchist in their title so look out for
Generally if you fail to find a local group by any of these methods then there is not one worth finding! You can also use the internet to try and find local groups but very often small local groups will not have any internet presence at all. You could email any national groups that are in your country and ask them but keep in mind that they also may not know of any local group or if they disagree with them may not tell you how to contact them.
Once you have located a group you will want to evaluate it to see if its worthwhile being involved in and what sort of contribution you can make to it.
While doing all this you should also start to engage is activity by yourself in case you don't find anyone else. This should also give other isolated anarchists a way of finding you.
There are things that an isolated individual can do to promote anarchism and find other anarchists. These include