I've decided it's about time I had a serious attempt at this game my concern is with experience penalties when mismatched levels etc., does it tell you that you have/will have a penalty active when it occurs ?

Yes, I think so, but it's not something that should ever come up really because it doesn't take much effort to avoid multiclass penalties.

I think with the party I've created I won't have the problem cos I'm not getting exotic

Aasimar Paladin(x)/Fighter(4)
Human Monk
Shield Dwarf Battleguard(x)/Fighter(4)
Halfling Rogue
Human Sorcerer
Moon Elf Wizard

Well, you'll have no problems there at all then.

By the way, you can avoid xp penalties with clever level squatting. For example, consider a drow sorc 27/paladin/monk/cleric. You might think it would suffer a 60% xp penalty, but actually it can be minimized it to almost nothing:
Female drow have cleric as their favored class, so it won't count against you. Take that level first.
Then take the 27 sorcerer levels. You'll still have no xp penalty.
At 465 000 xp, you'll be permitted to level up, but don't do it yet! Wait until you have 490 000 xp, then take the paladin level. Now you have a 20% xp penalty.
At 496 000 xp you'll get your final level (monk). Now you have a meaningless 40% xp penalty.

At the end, you've suffered only 20%*6000 = 1200 xp wasted.

Experience penaltys won't affect the average party level much.
The characters with the XP penalties will level up slower, but the others will level up faster because of the dynamic XP calculation.

@Darkstrider: I wouldn't give fighter levels to the battleguard since it's important to get spells like animate dead, magic circle against evil, raise dead and heal as fast as possible. Since your characters won't probably reach more than level 17 you'll loose much.

I agree with kmonster that you want a single-classed cleric. A better choice for multiclassing would be the rogue since you don't need all that many skills. There are a lot of good options for multiclassing him, such as a ranger level if you want dual wielding, barbarian for the increased speed, 4 levels of fighter if you want weapon spec, 3 levels of mage if you want to cast mirror image, etc...

I agree with JT that your rogue ist the best class for multiclassing.

But you don't have to multiclass him, unlike in 2e games, rogues get better and better with levels.

They gain + 1-6 sneak attack damage for every 2 levels and extra feats every 3 levels starting at level 10.

So your thief won't be really stronger when you give him other levels, it's rather a roleplaying choice.