Hi! 
What do you mean by metal hydride?
Sometimes, hydrogen can gain an electron instead of donating one to become a cation/proton. This hydrogen becomes an anion, having a -1 charge, and we call it a hydride.
Now, hydrogen atoms don't have a lot of charge density - in fact, a hydride has twice as much negative charge density as it does positive - so this makes them not very stable, with most hydride compounds reacting with moisture in the air.
As a result, they're particularly hard to make, and the only instance you're likely to see them in as attached to metal cations (typically the first two row metals, such as Ca, Li, Ba, Na, etc., but you can also get transition metal hydrides). Since we usually only see hydrides forming with metal, we also give them the name metal hydrides.
As for actually distinguishing if a hydrogen is in its cationic or anionic form, it's actually very easy! If you're given a compound, and it's just a hydrogen atom and a metal, the metal is always going to be the cation, so the hydrogen must be a hydride. In any other context (particularly in the case of carbon, oxygen, nitrogen and sulfur!), hydrogen doesn't like to be a hydride, and so you should assume that if it were to be assigned a charge, it would be +1.
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