How to know if your struts are bad?

I have a 2009 ford focus I had to change the front struts a few years back because i was ran into a side of a curb really hard. Anyway My car as been making a shaking noise for he past few months, i thought it was because he had to be realigned but I think there could be a bigger problem. Also there as been no excessive bouncing, noise dives, or leaking… What else could this be?

When you ran over that curb, you didn't hurt the struts -- they're just glorified shock absorbers, for potholes and other up and down movement. What you likely did was warp a steering part. I'd say take it to a "front end shop", and have the steering and front suspension examined.

There are too many interconnected parts to make a guess. Also, as a result, damage incurred on one side can effect the other. It needs to be put up in the air and everything checked.
tires/wheels/wheel bearings/suspension and steering gear. In some cases, front drive/half shafts and c/v joints maybe effected.
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to answer the Q, app for shocks and struts,
lean on front of car with all your weight. Hold it down. Let off quickly. Iffin she rises, then settles once to normal ride height, prolly good. Bouncing up and down more than i said *maybe* cause to Q the struts/shocks. Do note any unusualy noises. And keep just a finger on the nose of the car when you let go. You want to feel for any binding vibrations. Noises and vibrations may not be directly related to the struts/shocks. But it is a clue took look closer at them.
test will be app to the rear of a car as well.
most national chain stores like les schwab, sears, firestone, and the like offer a free inspection.
an alignment would be good good thing iffin the tires have unusualy wear patterns. And those chain stores can't only inspect/reapir/but set the alignment as well. Any repair save a wheel/tire/wheel bearing, and it likely will need an alignment anyway. Requirement/recommended actually.
respect,
i disagree on the generic idea that no damage can happen to a strut upon curb impact. Iffin a controller arm was bent, that would/could allow for the strut to bind internally. Can damage upper mounts and strut rod end. I've actually seen that. But that platform had very obvious steering/driveablility issues. May not be the case here.

Check your brake rotors and see if they need machining.