1 - You no longer have romantic time together, just the two of you
If you have noticed that you 2 are spending less and less time together (just the two of you), it may be a bad sign, especially if accompanied by his inviting friends every time you 2 are going out. If your time together has changed into a group outing, it is a clear sign he's ready to leave you. This is particularly the case if you notice that his eyes wonder towards other ladies in the group, or if he is acting flirtatious with any of them.
The breakup wish may still be in his subconscious, but it's there if he acts this way.
2 - He changes his looks
If your man suddenly looks much better, or different, you may have good reason to worry. For example; let's say your man is the casual guy, someone whose attire is limited to T-shirt, jeans and trainers; if all of a sudden he starts to buy designer clothes, for example, well he's clearly trying (subconsciously or consciously) to meet new (and different from you) women, or simply trying to impress another woman in particular. The same if he's just bought cologne whilst until then he wasn't even wearing a deodorant!
On the other hand, when a guy is comfortable in a relationship he let's himself go, although nowadays, with the pressure to look good or at least be fit, this may no longer be the case. But since this is the human tendency (namely that when we are single or think single we 'special' care of ourselves, whilst when we are comfortable in a relationship we don't), if your man is starting to take 'special 'care of himself in any way, it is a warning sign suggesting he's acting or thinking 'single' again.
2 - He's in a bad mood and/or overly critical of you
Do you feel like he's no longer interested in what you have to say, no matter what it is? Or do you feel that when you tell a great, funny story everybody is entertained whilst he looks bored and absent-minded? Or do you complain about some stranger treating you badly and all you hear from him is that he's taking the stranger's side? Do you feel like he's started to criticise you for no real reason, or become judgemental against you, whilst before he used to praise you or compliment you? Do you find that he's become snappy, bad-tempered and moody? If this is the case in your relationship, this could well be a sign that he's ready to dump you. You feel he has developed contempt towards you or the relationship and you are probably right (if the above behavioural signals are in place). The reason why he's psychologically torturing you this way is because he's acting out his frustration. He may feel bored or frustrated in the relationship but incapable to change things; when this is the case a man will often act out in these ways; the key is not to take it personally because you could be the most beautiful, the nicest, the kindest, the most charming woman on the planet and he would still act out in this manner if he's psychologically thinking 'single' again, or if he's feeling powerless or frustrated within the relationship (often for his own psychological reasons which have nothing to do with you).
4 - He's no longer available.
Has he suddenly become really busy at work that he finds less and less time for you? Or, are you suddenly the one who makes the effort to maintain contact? Are you the ones making more phone calls to him than he makes to you? Is he replying to your text messages or emails with unusual delays? Is he not returning your calls as quickly as he used to, if at all? Has he cancelled a few dates with you lately? Or, is he no longer planning anything in the future with you (holidays, living arrangements, and so on) and avoiding the subject when you bring it up? These signals are all clues that he's trying to 'get out of the relationship', subconsciously or even consciously; let's not forget that a guy finds it very hard to split up with any girl in person, face to face, so he resorts to these types of behaviour instead.
5 - Body language changes
Despite what a guy may be telling you, watch his body language when he's with you, talking or listening to you; body language signals such as shrugging his shoulders rather than answering you, or giving you a pathetic one-shoulder hug (this is a sign showing that he's subconsciously distancing himself from you), or if he pats you on the back whilst you 2 are hugging (this is a sign showing that he is subconsciously or consciously uncomfortable in hugging you).
If the above signals are cropping up in the relationship, rather than being upset or closings up (which are both understandable reactions) you need to establish or re-establish openness in your relationship, without any form of censorship or criticism; this is a good method to avoid an otherwise inevitable breakup and to restore love in the relationship.