The present perfect is most commonly used in three ways

1. To describe a life experience, I have eaten snake.
2. To describe repeated, discreet actions that have continued up to the present moment, We have received many phone calls supporting our cause.
3. To describe actions recently completed, The doors have closed.

Thoughts are discreet events. You can be thinking something, but if you have a thought, by the time you describe it, it's complete.

Looking at the above list, the only one that fits is if you have certain repeated thoughts, that continue up to the present moment. Normally you will include some kind of adverb to describe the frequency.

A: So you think you should only serve white wine with fish?
B: I have often thought so, but I recently read you can pair something like a dry pinot noir with salmon. So I'll have to try it and let you know.

Otherwise use the simple past to indicate a complete thought, or the past progressive to indicate an ongoing thought, concurrent with some other action:

A: Did you find your keys in your coat pocket?
B: I was thinking I left them there, but then I saw them on the mantle.

You can also use the past perfect:

I had thought I left my keys in my coat, but then I found them in my jeans.

The present perfect is most commonly used in three ways

1. To describe a life experience, I have eaten snake.
2. To describe repeated, discreet actions that have continued up to the present moment, We have received many phone calls supporting our cause.
3. To describe actions recently completed, The doors have closed.

Thoughts are discreet events. You can be thinking something, but if you have a thought, by the time you describe it, it's complete.

Looking at the above list, the only one that fits is if you have certain repeated thoughts, that continue up to the present moment. Normally you will include some kind of adverb to describe the frequency.

A: So you think you should only serve white wine with fish?
B: I have often thought so, but I recently read you can pair something like a dry pinot noir with salmon. So I'll have to try it and let you know.

Otherwise use the simple past to indicate a complete thought, or the past progressive to indicate an ongoing thought, concurrent with some other action:

A: Did you find your keys in your coat pocket?
B: I was thinking I left them there, but then I saw them on the mantle.

You can also use the past perfect:

I had thought I left my keys in my coat, but then I found them in my jeans.

(Edit) The idiomatic way to say this is with the simple past:

A: Here are your keys, on the table in the living room.
B: Aha! I thought so.
A: But you didn't know so, otherwise you would have looked there first.

(Note the difference between thinking something and knowing it.)

The present perfect is most commonly used in three ways

1. To describe a life experience, I have eaten snake.
2. To describe repeated actions that have continued up to the present moment, We have received many phone calls supporting our cause.
3. To describe actions recently completed, The doors have closed.

Thoughts are discreet events. You can be thinking something, but if you have a thought, by the time you describe it, it's complete.

Looking at the above list, the only one that fits is if you have certain repeated thoughts, that continue up to the present moment. Normally you will include some kind of adverb to describe the frequency.

A: So you think you should only serve white wine with fish?
B: I have often thought so, but I recently read you can pair something like a dry pinot noir with salmon. So I'll have to try it and let you know.

Otherwise use the simple past to indicate a complete thought, or the past progressive to indicate an ongoing thought, concurrent with some other action:

A: Did you find your keys in your coat pocket?
B: I was thinking I left them there, but then I saw them on the mantle.

You can also use the past perfect:

I had thought I left my keys in my coat, but then I found them in my jeans.

The present perfect is most commonly used in three ways

1. To describe a life experience, I have eaten snake.
2. To describe repeated, discreet actions that have continued up to the present moment, We have received many phone calls supporting our cause.
3. To describe actions recently completed, The doors have closed.

Thoughts are discreet events. You can be thinking something, but if you have a thought, by the time you describe it, it's complete.

Looking at the above list, the only one that fits is if you have certain repeated thoughts, that continue up to the present moment. Normally you will include some kind of adverb to describe the frequency.

A: So you think you should only serve white wine with fish?
B: I have often thought so, but I recently read you can pair something like a dry pinot noir with salmon. So I'll have to try it and let you know.

Otherwise use the simple past to indicate a complete thought, or the past progressive to indicate an ongoing thought, concurrent with some other action:

A: Did you find your keys in your coat pocket?
B: I was thinking I left them there, but then I saw them on the mantle.

You can also use the past perfect:

I had thought I left my keys in my coat, but then I found them in my jeans.