how can you tell who can use what items and what slots they will fit in? Thanks

1st indicator: Color. For example Mental Tool is violet and the mental tool mods are always violet too. Sadly there are lots of different items with similar colors still ...

2nd indicator: READ. You have to read the description. The item type of a mod is always in the headline. For example it will say " xyz for Precision Ranged Weapons". Which means the mod is for Precision ranged only. If it says "xyz for Ranged Weapon" the mod is for ANY ranged.

thanks man

The best way is to simply click an equipment slot and the inventory will filter all suitable items for that for you.

In the near future there will be a major game update with UI overhaul that includes among other things improved item identification techniques. Please stay tuned.