Hi,

Similar to chiral molecules, there is a minimum number of different groups to draw a pair of diastereomers - that is you need 2 different groups on each side of the alkene (not all 4 have to be different from each other.

As a result, when you are looking for a cis/trans alkene, look for something where you have a configuration like


A_____A
_=====
B_____B

or something like this.

I will draw out two cases on this question as well.

https://ibb.co/5WKJhFD

As CH3CH2CCl=CClH has AB and AB on each side (ie two different groups), it can be drawn as cis/trans, while 2-methyl-2-butene has AA on one side, meaning it doesn't matter if you switch AB on the other side, the molecule can't be cis or trans (you need a point of reference to be cis to something).

As for how you can convert the condensed Lewis structure of CH3CH2CCl=CClH to the skeletal structure, I usually start with a CH3 and draw from there, because you know the CH3 is the end of your molecule. It gets tougher with branch points but you can then trace the spine of the molecule from there. Practice definitely helps with this.