If I had a penny for every woman who asks, "Is he cheating on me"? when they already know what the answer is...
Still I'm going to help you out here and give you some obvious clues to look for and we'll talk about what to do with them.

He doesn't care that much about you:
This doesn't really mean he will , but a guy is more likely to cheat on you if he doesn't like or respect you. Most people are just dumb enough to think someone loves them based off of hearing three sweet words. It doesn't matter that your partner treats you as less than the bug he just stepped on as long as he says "I love you", that's all that matters. Right?

He's always been a one trick pony in the bed room and suddenly wants to pull out some new moves :
Okay, I wouldn't take this one too seriously unless it's being accompanied by other more suspicious behavior. Chances are he just found your secret Cosmo mag stash. Seriously, you should burn those things.

He's been to your home several times but refuses to take you to his or even tell you where he lives:

Yeah, he's cheating alright. With you! So how's it feel to be a home wrecker? Just in case that one went over your head, HE'S MARRIED! Oh and FYI it does not count as being cheated on when they are legally married to someone else. Gotta love it when some stupid girl is all like, "Oh my god! He's cheating on me with his wife!". *Insert face palm*

He starts a nonsense argument with you and then storms out of the house and leaves:
Those "fights" are just an escape route so he can go meet his lover. Yes, every couple has their disagreements but I'm talking unusual stuff like starting a fight over the cat when you don't even have a cat.

He's more attentive to your needs or starts buying you lots of gifts:

All I saw was "lots of gifts"
Seriously , that supposed to be a "sign of guilt". You could either confront him or introduce him to your long lost cousin and get double the presents.

He's happy to leave you:
If he's normally a dirty downer personality around you but dressed to kill and smelling as if he bathed in Axe body spray (I'd dump him just for wearing that junk) and smiling when he leaves out the door, something is not right.

He begins accusing you without any realistic proof:
This is a favorite move of many cheaters. It helps them to justify their actions. If they can make themselves believe that you are the one who is being unfaithful, it will help them feel better and then that way they can claim that what they did was "all for revenge".

He claims that all men cheat:
Translation: " I will cheat on you". Good thing he informed you ahead of time. Hopefully you're smart enough to run the other way. All men do not cheat. This is a favorite lie of serial cheaters to justify their actions. They want you to believe this is normal behavior for men so that you give in and accept it but it is not. It's only normal for him.

He never has time for you, but always has time for his friends:
Every man needs his guy time just as every woman needs her girl time but there should always be room for us time. If he's not willing it's because he's spending his us time with someone else.

He talks in a low voice on the phone every time a certain person calls or leaves the room so you can't hear him:
He's talking lovey dovey with his lover

He suddenly becomes hostile and abusive:
People always assume that domestic violence always starts at the beginning of a relationship and that is far from being the truth. A husband can be very gentle at the start until he falls in love with someone else and feels resentment toward his wife. This resentment can come from feeling trapped in his marriage when he wants to be else where. Still, this does not give any reason whatsoever for him to hit or make you feel bad. You have no control over where his heart decides to lead him.

He no longer shows interest in you or the children he once loved and adored and he shows no concern about your future together:
His priorities and what is important to him has changed and none of those things includes you.

He is unwilling to work out problems together:

No couple is perfect. A husband and wife should team up together to sort our their disagreements. If you're the only one putting forth any effort to fix things and he's not, its because he doesn't care what happens.

He's never available and hard to get a hold of:
He never picks up the phone when you call but if its anyone else he'll trip all over his own feet to answer the answer the darned thing. He can't possibly let his lover find out about you.

He constantly avoids saying "I love you" over the phone:
A cheating man might be very comfortable with returning your "I love you" when his lover is not around but give you a lame "Uh huh" or say "I love me too" and play it off as a joke if he/she is with him.
Now, I know that some people will come up saying that some guys are embarrassed to say these things in front of their guy friends. That may be true for the less mature little boys but most grown men have no fear in telling their woman exactly how they feel no matter who is listening.

Your gut instinct says so:
You know your husband/boyfriend better than anyone else and you should also know when somethings not right with him.

Now that you have the obvious clues, what should you do? That ones up to you. Personally I believe once a cheater always a cheater. I have heard cases of second chances being given, marriages being made stronger and affairs ending and never happening again but those out comes seem to be greatly outweighed by the negatives.

Most people say that cheaters think that being given a second chance is being given permission to cheat again. Actually that reminds me very much of how a child would react if he didn't eat his peas but you gave him a cookie anyway. He's not going to take you seriously after that. Honestly though if a grown man needs to have his hand held and be shown right and wrong as if he were a toddler, you need to drop that sucker anyway and find one who is able to think for himself.

There may not be anything for you to work out. If he has obviously fallen for someone else and shows signs that he no longer cares about you and yours, then it's safe to say its over for good. You can't fix something that they want to stay broken.