I used to teach in a community where forced marriages weren't unheard of. The problem is, it can be difficult to tell as within the particular cultural group I have worked with, girls don't really have any expectation of choosing their own husbands anyway. Nor do the marriages happen in the UK. Typically the girl is sent 'home' on holiday, to see ageing relatives or to attend a funeral and does not come back until she is married.
In terms of what to look out for it can be very difficult as often they are simply not told they are going back to the home country for a visit and when they do they are often isolated in villages far away from modern communication technology and are given a fait accompali. The only thing we could do was help the girl if she found the tickets and suspected something may be up. (I made many phone calls to Southall Black Sisters during my years working there.) Of course, it is entirely possible, and not infrequent, for the trip to be short notice for legitimate reasons (funerals, ill relatives) or for family holidays. Finding tickets with their name on them was in no way an indication of a forced marriage in the offing.
It's also dangerous to assume it's only young girls who are subject to family coercion to get married. The one case I had most to do with was a colleague who was in her early 20s. Her father controlled her bank account, kept her bank card, and paid for her wedding out of her own salary. She didn't want to get married, strongly objected to it, but gave in in the end because, to her mind, the pain of being ostracized by the family was too much to bear. She agreed to give it a try, but went to the GP first (with me in tow for moral support) to get the depo provera injection first so she wouldn't get pregnant and wouldn't be seen to be taking contraceptives.
Apart from my colleague who managed to divorce her husband after a year, most of the girls who went on holiday and came back married seem to just shrug. They may not have wanted to get married, may not like their husband, but they seemed fatalistic about the whole thing since they never expected to choose their own husband in the first place.