The solicitor should have their file notes, as @VanGoghsDog says. However, it may depend how long ago the will was drawn up. If it was a number of years they may not, some solicitors destroy them automatically after 5 years.
Anyone who benefits from a will should not really be involved at all in the witnessing of it and a solicitor should know that. That would be seen as not good practice.
Unfortunately, anyone can change their will as it suits them, so notes of possible intent pre-this will are just that. The will itself over-rides those if it was properly made.
Do you know exactly what it says, who the estate was left to, who the executors are and what it amounts to? That is important.
If it is a small estate, it us unlikely to be worth challenging legally. If you did challenge and won, the estate would likely have to bear the costs which could eat up the estate. If you challenged and lost, you would have to bear the costs of the challenge and possibly the defence yourself. Bear in mind that if you required a barrister that could escalate costs. However, not all cases end up in court, many are settled by negotiation. For example if you are really upset about some personal belongings your solicitor can write and request that the beneficiary of the estate considers passing those items to you as keepsakes of your relationship with the deceased. They might well do so if it avoids a court case. If this person is the sole beneficiary and executor they have two years in which they can choose to vary the will -ie they could choose to make other bequests if they felt someone had been left out. A solicitor can find a way to suggest that as a negotiation.
If it is a significant estate, it might be worth pursuing the matter in court if you feel really strongly. You need to weigh up your evidence rather than your feelings about what you think happened. A solicitor would be very silly to put their professional reputation at risk by colluding with someone who was coercing a vulnerable person. A solicitor has to be seen to act in the best interests of their client and should take particular steps to assure themselves that- at the point the instructions are given to them by the client- their client has the capacity to make the will and is doing so of their own free will without any undue influence. Alzheimers or dementia do not preclude someone from having the capacity to make a will. That person may have periods of lucidity where they are quite clear in their thinking. Witnesses should not be executors who are going to benefit. You can see how the notes made during this process by the solicitor can be so revealing- they should contain details of how the solicitor ascertained capacity and free will. The solicitor, if they have acted improperly can be sued and the firm will have insurance to cover this should they be found to have acted improperly.
Evidence that would carry some weight might be medical evidence from the time the will was written, or if you are a spouse, or child of the deceased and were promised an inheritance which you can prove you relied and depended on-that could be seen as carrying some weight. Fir instance if you never bought a property of your own and lived with the deceased and invested in theirs over the years on the understanding they would be leaving a share to you that could be seen as you depending on an expectation of inheritance. You have to be a very close relative with proof of the dependence and expectation to claim this.
A previous will in your favour would only be evidence taken into account if for whatever reason the final will was found to be invalid- in which case the immediate previous will would be re-instated as long as that was valid. If a will is written by a solicitor who has taken the appropriate steps to ascertain capacity and free will of the client, and the will is properly drawn up, signed and witnessed it would be unlikely to be found invalid.